SRAM patcher: add SRAM virtual address patcher

Add omap_sram_patch_va(), which patches a virtual address into SRAM
code at runtime.  This will be used in a future patch series to implement
part of multiboot support for OMAP2/3.

If CONFIG_OMAP_DEBUG_SRAM_PATCH is defined (the default), the code
will be very careful to ensure that the target location to patch is
valid.  It will only overwrite a location if the location contains a
32-bit magic number, defined as SRAM_VA_MAGIC.  Also, defining DEBUG
at the top of the file and enabling CONFIG_DEBUG_LL will log patch
locations and data via printk.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>