can: m_can: Kconfig: convert the into menu

Since there is more than one base driver for the m_can core, let's
convert this into a menu.

Link: https://lore.kernel.org/r/20201130133713.269256-4-mkl@pengutronix.deReviewed-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
-config CAN_M_CAN
+menuconfig CAN_M_CAN
 	tristate "Bosch M_CAN support"
 	help
 	  Say Y here if you want support for Bosch M_CAN controller framework.
 	  This is common support for devices that embed the Bosch M_CAN IP.
 
+if CAN_M_CAN
+
 config CAN_M_CAN_PLATFORM
 	tristate "Bosch M_CAN support for io-mapped devices"
 	depends on HAS_IOMEM
-	depends on CAN_M_CAN
 	help
 	  Say Y here if you want support for IO Mapped Bosch M_CAN controller.
 	  This support is for devices that have the Bosch M_CAN controller
 	  IP embedded into the device and the IP is IO Mapped to the processor.
 
 config CAN_M_CAN_TCAN4X5X
-	depends on CAN_M_CAN
 	depends on SPI
 	select REGMAP_SPI
 	tristate "TCAN4X5X M_CAN device"
@@ -23,3 +23,5 @@ config CAN_M_CAN_TCAN4X5X
 	  Say Y here if you want support for Texas Instruments TCAN4x5x
 	  M_CAN controller.  This device is a peripheral device that uses the
 	  SPI bus for communication.
+
+endif