ARM: OMAP2+: clock: add support for indexed memmaps

Using indexed memmaps is required for isolating the actual memory access
from the clock code. Now, the driver providing the support for the clock IP
block provides the low level routines for reading/writing clock registers
also.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>