spi: add support for mediatek spi-nor controller

This is a driver for mtk spi-nor controller using spi-mem interface.
The same controller already has limited support provided by mtk-quadspi
driver under spi-nor framework and this new driver is a replacement
for the old one.

Comparing to the old driver, this driver has following advantages:
1. It can handle any full-duplex spi transfer up to 6 bytes, and
   this is implemented using generic spi interface.
2. It take account into command opcode properly. The reading routine
   in this controller can only use 0x03 or 0x0b as opcode on 1-1-1
   transfers, but old driver doesn't implement this properly. This
   driver checks supported opcode explicitly and use (1) to perform
   unmatched operations.
3. It properly handles SFDP reading. Old driver can't read SFDP
   due to the bug mentioned in (2).
4. It can do 1-2-2 and 1-4-4 fast reading on spi-nor. These two ops
   requires parsing SFDP, which isn't possible in old driver. And
   the old driver is only flagged to support 1-1-2 mode.
5. It takes advantage of the DMA feature in this controller for
   long reads and supports IRQ on DMA requests to free cpu cycles
   from polling status registers on long DMA reading. It achieves
   up to 17.5MB/s reading speed (1-4-4 mode) which is way faster
   than the old one. IRQ is implemented as optional to maintain
   backward compatibility.
Signed-off-by: Chuanhong Guo <gch981213@gmail.com>
Link: https://lore.kernel.org/r/20200306085052.28258-3-gch981213@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/Kconfig

@@ -443,6 +443,16 @@ config SPI_MT7621
 	help
 	  This selects a driver for the MediaTek MT7621 SPI Controller.
 
+config SPI_MTK_NOR
+	tristate "MediaTek SPI NOR controller"
+	depends on ARCH_MEDIATEK || COMPILE_TEST
+	help
+	  This enables support for SPI NOR controller found on MediaTek
+	  ARM SoCs. This is a controller specifically for SPI-NOR flash.
+	  It can perform generic SPI transfers up to 6 bytes via generic
+	  SPI interface as well as several SPI-NOR specific instructions
+	  via SPI MEM interface.
+
 config SPI_NPCM_FIU
 	tristate "Nuvoton NPCM FLASH Interface Unit"
 	depends on ARCH_NPCM || COMPILE_TEST

drivers/spi/Makefile

@@ -62,6 +62,7 @@ obj-$(CONFIG_SPI_MPC52xx_PSC)		+= spi-mpc52xx-psc.o
 obj-$(CONFIG_SPI_MPC52xx)		+= spi-mpc52xx.o
 obj-$(CONFIG_SPI_MT65XX)                += spi-mt65xx.o
 obj-$(CONFIG_SPI_MT7621)		+= spi-mt7621.o
+obj-$(CONFIG_SPI_MTK_NOR)		+= spi-mtk-nor.o
 obj-$(CONFIG_SPI_MXIC)			+= spi-mxic.o
 obj-$(CONFIG_SPI_MXS)			+= spi-mxs.o
 obj-$(CONFIG_SPI_NPCM_FIU)		+= spi-npcm-fiu.o