Runtime constants: use runtime-computed SMS base

Create a global variable, omap2_sms_base, that is initialized with the
appropriate SMS base address at runtime during architecture
initialization.  Convert users of the preprocessor define
OMAP2_SMS_BASE to use the runtime-computed address, and get rid of
OMAP2_SMS_BASE.  Create sms_{read,write}_reg() to handle register
access to these functions, located in arch/arm/mach-omap2/sdrc.h.
Define the only SMS register that we currently use, SMS_SYSCONFIG, in
include/asm-arm/arch-omap/sdrc.h, so it can be included in assembly
language files.

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