ARM: OMAP2: Powerdomain: Add base OMAP2/3 powerdomain code

This patch creates an interface to the powerdomain registers in the
PRM/CM modules on OMAP2/3.  This interface is intended to be used by
PM code, e.g., pm.c; not by device drivers directly.

Each powerdomain will be defined in later patches as static
structures.  Also defined are dependencies between powerdomains,
used for adding and removing PM_WKDEP and CM_SLEEPDEP bits.  The
powerdomain structures are linked into a list at boot by
pwrdm_register(), similar to the OMAP clock code.

The patch adds a Kconfig option, CONFIG_OMAP_DEBUG_POWERDOMAIN, which
when enabled will emit verbose debug messages via pr_debug().

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