This patch adds new at91 main oscillator clock implementation using common
clk framework.

If rate is not provided during clock registration it is calculated using
the slow clock (main clk parent in this case) rate and MCFR register.
Signed-off-by: NBoris BREZILLON <b.brezillon@overkiz.com>
Acked-by: NMike Turquette <mturquette@linaro.org>
Signed-off-by: NNicolas Ferre <nicolas.ferre@atmel.com>