[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>