ARM: OMAP2+: hwmod: use init-time function ptrs for enable/disable module

The enable/disable module functions are specific to SoCs with
OMAP4-class PRCM.  Rather than use cpu_is* checks at runtime inside
the enable/disable module functions, use cpu_is at init time to
initialize function pointers only for SoCs that need them.

NOTE: the cpu_is* check for _enable_module was different than
      the one for _disable_module, and this patch uses
      cpu_is_omap44xx() for both.

Signed-off-by: Kevin Hilman <khilman@ti.com>
[paul@pwsan.com: moved soc_ops function pointers to be per-kernel rather than
 per-hwmod since they do not vary by hwmod; added kerneldoc]
Signed-off-by: Paul Walmsley <paul@pwsan.com>