OMAP4: cm: Add two new APIs for modulemode control

In OMAP4, a new programming model based on module control instead
of clock control was introduced.
Expose two APIs to allow the upper layer (omap_hwmod) to control
the module mode independently of the parent clocks management.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Rajendra Nayak <rnayak@ti.com>
[paul@pwsan.com: renamed 'omap4_cm_' fns to 'omap4_cminst_'; cleaned up
 kerneldoc]
Signed-off-by: Paul Walmsley <paul@pwsan.com>