clk: xgene: Add PMD clock

Add X-Gene PMD clock support.

PMD clock is implemented for a single register field.
  Output rate = parent_rate * (denominator - scale) / denominator
with
  - denominator = bitmask of register field + 1
  - scale = values of register field

For example, for bitmask is 0x7, denominator will be 8 and scale
will be computed and programmed accordingly.

Signed-off-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>