OMAP2: clock: add DPLL autoidle support

Add the necessary code and data to allow the clock framework to enable
and disable the OMAP2 DPLL autoidle state.  This is so the direct
register access can be moved out of the mach-omap2/pm24xx.c code, and other
code that needs to control this (e.g., CPUIdle) can do so via an API.
As part of this patch, remove the pm24xx.c code that formerly wrote
directly to the autoidle bits.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@ti.com>
Tested-by: Rajendra Nayak <rnayak@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>