Runtime constants: use runtime-computed system control module base

Create a global variable, omap2_ctrl_base, that is initialized with
the appropriate control module base address at runtime during
architecture initialization.  Convert users of the preprocessor define
OMAP2_CTRL_BASE to use the runtime-computed address, and get rid of
OMAP2_CTRL_BASE.  Create ctrl_{read,write}_reg() to handle register
access to these functions - these live in a newly-created file,
arch/arm/mach-omap2/control.h.  Add the control register defines that
we currently use into a newly-created file,
include/asm-arm/arch-omap/control.h, where they can be imported into
assembly-language code.

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