Runtime constants: use runtime-computed SDRC base

Create a global variable, omap2_sdrc_base, that is initialized with
the appropriate SDRC base address at runtime during architecture
initialization.  Convert users of the preprocessor define,
OMAP2_SDRC_BASE, to use the runtime-computed address.  Create
sdrc_{read,write}_reg() to handle register access to these functions -
these live in a newly-created file, asm/arch/mach-omap2/sdrc.h.  Move
the SDRC register definitions into include/asm-arm/arch-omap/sdrc.h,
so they can be included in assembly language files.

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