sh: clkfwk: Provide a generic clk_set_rate_ex() path for root clocks.

In the case of root clocks (such as clkin oscillators, extal, etc.), the
rate information is entirely platform dependent and needs to be lazily
set and propagated from the platform code. This provides a method for
establishing the rate update on these types of clocks that define no
set_rate() op of their own.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>