[ARM] add CONFIG_HIGHMEM option

Here it is... HIGHMEM for the ARM architecture.  :-)

If you don't have enough ram for highmem pages to be allocated and still
want to test this, then the cmdline option "vmalloc=" can be used with
a value large enough to force the highmem threshold down.

Successfully tested on a Marvell DB-78x00-BP Development Board with
2 GB of RAM.
Signed-off-by: Nicolas Pitre <nico@marvell.com>

arch/arm/Kconfig

@@ -915,6 +915,23 @@ config NODES_SHIFT
 	default "2"
 	depends on NEED_MULTIPLE_NODES
 
+config HIGHMEM
+	bool "High Memory Support (EXPERIMENTAL)"
+	depends on MMU && EXPERIMENTAL
+	help
+	  The address space of ARM processors is only 4 Gigabytes large
+	  and it has to accommodate user address space, kernel address
+	  space as well as some memory mapped IO. That means that, if you
+	  have a large amount of physical memory and/or IO, not all of the
+	  memory can be "permanently mapped" by the kernel. The physical
+	  memory that is not permanently mapped is called "high memory".
+
+	  Depending on the selected kernel/user memory split, minimum
+	  vmalloc space and actual amount of RAM, you may not need this
+	  option which should result in a slightly faster kernel.
+
+	  If unsure, say n.
+
 source "mm/Kconfig"
 
 config LEDS