OMAP4: hwmod: Replace CLKCTRL absolute address with offset macros

The CLKCTRL register was accessed using an absolute address.
The usage of hardcoded macros to calculate virtual address from physical
one should be avoided as much as possible.
The usage of a offset will allow future improvement like migration from
the current architecture code toward a module driver.

Update cm_xxx accessor, move definition to the proper header file and
update copyrights.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Rajendra Nayak <rnayak@ti.com>
Cc: Todd Poynor <toddpoynor@google.com>
[paul@pwsan.com: renamed 'omap4_cm_' fns to 'omap4_cminst_'; removed empty
 fn prototype section from cm44xx.h; incorporated comments from Todd;
 documented some functions]
Signed-off-by: Paul Walmsley <paul@pwsan.com>