clk: bcm2835: Minimise clock jitter for PCM clock

clk: bcm2835: Minimise clock jitter for PCM clock

Fractional clock dividers generate accurate average frequencies but
with jitter, particularly when the integer divisor is small.

Introduce a new metric of clock accuracy to penalise clocks with a good
average but worse jitter compared to clocks with an average which is no
better but with lower jitter. The metric is the ideal rate minus the
worse deviation from that ideal using the nearest integer divisors.

Use this metric for parent selection for clocks requiring low jitter
(currently just PCM).

Signed-off-by: Phil Elwell <phil@raspberrypi.org>