OMAP3 clock: add "prcm_mod" field to OMAP3xxx clocks

Annotate each OMAP2xxx real hardware clock controlled by the PRCM with
the PRCM module offset.

A subsequent patch will use this to simplify register addressing in the
struct clk.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>