arm: at91: clock: Add the generated clock support

Some peripherals may need a second clock source that may be different
from the system clock. This second clock is the generated clock (GCK)
and is managed by the PMC via PMC_PCR.

For simplicity, the clock source of the GCK is fixed to PLLA_CLK.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>