clk: at91: rework main clk implementation

AT91 main clk is a clk multiplexer and not a simple fixed rate clk as
currently implemented.

In some SoCs (sam9x5, sama5, sam9g45 families) this multiplexer can
choose among 2 sources: an internal RC oscillator circuit and an
oscillator using an external crystal.

In other Socs (sam9260, rm9200 families) the multiplexer source is
hardcoded to the external crystal oscillator.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>