clk: oxnas: Add OX820 Gate clocks

Add support for the Oxford Semiconductor OX820 SoC gate clocks
along the OX810SE SoC support.
This rework on concerns the gate clocks since they are different.
Future PLL handling code will be added for OX820.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/20161005150752.22618-6-narmstrong@baylibre.com