msm: clock-8974: Add support for the HDMI PHY PLL

The mdss_extpclk clock is sourced from the HDMI PHY PLL, which
is programmed by the HDMI driver. Add support for this PLL, and
link up the PLL with the exptclk via the set_rate op in the RCG.

Change-Id: I642ce976db8f0bbce7e1bafcfce3b51680b1822a
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>