serial: sh: Use the clock framework to obtain clock config

serial: sh: Use the clock framework to obtain clock config

Since we now have clock driver on the RCar Gen3 , obtain the clock
configuration using the clock framework functions. In case this
fails, fall back to the original code for pulling the clock config
directly out of OF.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>