ARM: OMAP2PLUS: Enable HIGHMEM

HIGMEM support in kernel is quite mature now and we have boards
like ZOOM, PANDA, SDP where 1 GB memories are installed. With
HIGHMEM disabled not all of the 1GB of RAM (only ~700MB) can be
accessed. Hence, enable HIGMEM to make use of the entire memory.

On the boards which doesn't have more than 768 MB memory, all
the memory is directly mapped in "lowmem" and highmem isn't
exercised. Hence, there should be no impact by enabling HIGHMEM
for boards that do not need it.

Tested on OMAP4460 Panda-ES.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Tested-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>