meson: clk: Add support for clock gates

This patch adds support for the meson8b clock gates. Most of
them are disabled by Amlogic U-Boot, but need to be enabled
for ethernet, USB and many other components.

Signed-off-by: Alexander Müller <serveralex@gmail.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1472319654-59048-7-git-send-email-serveralex@gmail.com