clk: at91: add new DT lookup function

Add a new DT lookup function to lookup for PMC clocks.

Note that the #ifndef AT91_PMC_MOSCS section will be removed once all the
platforms are converted.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>