ARM: OMAP2+: clockdomains: make {prm,cm}_clkdm common

The PRM and CM implicit clockdomains will soon be used by OMAP44xx.
So, make them common to OMAP2+ and modify the OMAP4 clockdomains code
so use of these clockdomains doesn't crash the system.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Rajendra Nayak <rnayak@ti.com>