[ARM] mm: move vmalloc= parsing to arch/arm/mm/mmu.c

[ARM] mm: move vmalloc= parsing to arch/arm/mm/mmu.c

There's no point scattering this around the tree, the parsing
of the parameter might as well live beside the code which uses
it.  That also means we can make vmalloc_reserve a static
variable.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>