MLK-16581-1 phy: mixel-lvds: Get PHY clock rate before setting it's rate

Due to i.MX8 clock issue, we need to get PHY clock rate
before setting it's rate when system resumes back from
PM sleep mode, otherwise, we'll fail to set the clock rate.
So, this is a workaround and it can be removed when
the clock issue is properly fixed.

Signed-off-by: Liu Ying <victor.liu@nxp.com>
(cherry picked from commit f9709fbf)