ARM: OMAP: Split sram.h to local headers and minimal shared header

Most of the defines are specific to omap1 and omap2+,
and should be in the local headers. Only minimal function
prototypes need to be shared.

As discussed on linux-arm-kernel, we want to avoid
relative includes for the arch/arm/*omap* shared code:

http://www.spinics.net/lists/linux-omap/msg80520.html

So this patch re-adds a minimal plat/sram.h.

The new plat/sram.h must not be included from drivers,
that will break build for omap2+ CONFIG_MULTIPLATFORM.

Note that this patch temporarily adds two more
relative includes; Those will be removed in the
following patch.

Signed-off-by: Tony Lindgren <tony@atomide.com>