serial: sh-sci: clkdev updates for MSTP gating.

This fixes up some of the I/D/F clock ambiguity in the sh-sci driver.
The interface clock in most cases just wraps back to the peripheral
clock, while the function clock wraps in to the MSTP bits. As the logic
was somewhat inverted, this cleans that up, and also enables all CPUs
with SCI MSTP bits to match function clocks through clkdev lookup.

As a result, this gets rid of the clk string abuse on the sh side, and
the clock string will be killed off once the ARM code has had a chance to
sync up. This also enables MSTP gating on CPUs like 7786 which had never
wired it up before. Impacted CPUs are primarily all SH-Mobiles, SH7785,
and SH7786.

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