ARM: OMAP2: Add common register access for 24xx and 34xx

This patch adds common register access for 24xx and 34xx power
and clock management in order to share code between 24xx and 34xx.

Only change USB platform init code to use new register access, other
access will be changed in later patches.

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