drm: panfrost: add optional bus_clock

Allwinner H6 has an ARM Mali-T720 MP2 which required a bus_clock.

Add an optional bus_clock at the init of the panfrost driver.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190521161102.29620-2-peron.clem@gmail.com