clk: fractional-divider: support for divider bypassing

If the divider or multiplier values are 0 in the register, bypassing the
divider and returning the parent clock rate in clk_fd_recalc_rate().

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Michael Turquette <mturquette@linaro.org>
[mturquette@linaro.org: fixed commitlog typo]