ARM: OMAP: OCM RAM: use memset_io() when clearing SRAM

We currently treat the OCM RAM memory as memory that hangs off an
I/O bus, so use memset_io() to clear it rather than memset().
This also takes care of this sparse warning:

arch/arm/plat-omap/sram.c:199:9: warning: cast removes address space of expression

There is still one SRAM-related sparse warning, but it appears that
fixing it correctly will take some time and thought.

N.B., at some point, the OCM RAM IP block interface code should probably
be reimplemented as an MTD device or something similar under drivers/.

Signed-off-by: Paul Walmsley <paul@pwsan.com>