ARM: OMAP3: PRM: add support for legacy iomapping init

As the legacy clock data is being moved under clock driver, the
clock data will be using the same low level infrastructure for
register accesses. This requires the clk_memmaps to be initialized
properly. This patch adds a support hook to the PRM driver to
initialize the mappings.

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