usb: phy: qusb: Update ref_clk and ref_clk_src clock usage

GCC_QUSB_REF_CLK_EN register controls the enable/disable of the
DIFF/CML clock to the QUSB2 PHY. When using SE/CMOS clock, control
of the DIFF clk is not needed. ref_clk is optional clock but driver
is not marking ref_clk as NULL if it fails to get reference of this
clock. Hence query or update this clock reference only if devicetree
qusb2phy node is having reference to this clock.

ref_clk_src is mandatory to provide SE/CMOS or DIFF/CML based
reference clock. Hence make ref_clk_src as mandatory clock.

Also use qusb_phy_enable_clocks() API within qusb_phy_remove() API
for disabling QUSB2 PHY related clocks.

Change-Id: I697e5c718285449268d5bb32deff5d18acf0cc2a
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>