clk: meson8b: convert to platform_driver

This patch creates a proper platform_driver for the meson8b clock
controller. Use of CLK_OF_DECLARE is removed, and can be added back in
later if very early registration of some clocks is required.

Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>