ARM: u300: move the gated system controller clocks to DT

This moves the slow, fast, AHB bridge and "rest" clocks on
the U300 system controller over to registration from the
device tree.

Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>