ARM: OMAP4: clock: add support for determine_rate for omap4 regm4xen DPLL

Similarly to OMAP3 noncore DPLL, the implementation of this DPLL clock
type is wrong. This patch adds basic functionality for determine_rate
for this clock type which will be taken into use in the patches following
later.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>