clk: meson-gxbb: Export PWM related clocks for DT

Add the PWM related clocks in order to be referenced as PWM source
clocks.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1471870177-10609-1-git-send-email-narmstrong@baylibre.com