clk: armada-37xx-periph: Support changing clock parent and rate

Add support for changing clock rate and parent clock for Armada 37xx
peripheral clocks.

Only clocks which can be disabled (.can_gate is true) can have parent
or rate changed.

This is needed so that Turris Mox can change SPI clock in device tree.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Stefan Roese <sr@denx.de>