clk: pxa: fix pxa27x CCCR bit usage

Trivial fix to check the A bit of CCCR for memory frequency
calculations, where the shift of the bit index was missing, triggering a
wrong calculation of memory frequency.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Michael Turquette <mturquette@linaro.org>