clk: at91: Add sama5d4 sckc support

Starting with sama5d4, the crystal oscillator is always enabled at startup
and the SCKC doesn't have an OSC32EN bit anymore.

Add support for that new controller.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>