OMAP: Increase VMALLOC_END to allow 256MB RAM

This increases VMALLOC_END to 0x18000000, making room for 256MB
RAM with the default 128MB vmalloc region.

Note that after this patch there's no longer a hole between vmalloc
space and the beginning of IO space on omap2 as the first virtual
mapping starts at 0xd8000000.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Acked-by: Kevin Hilman <khilman@deeprootsystems.com>