usb: phy: qusb: Fix QUSB PHY sequence based on clk source

CLK_REF_SEL (BIT:7) of QUSB2PHY_PLL_TEST register selects clock source
with QUSB PHY as differential clock or single ended clock. Based on
this it is required to use QUSB PHY initialization sequence. This
change updates QUSB PHY initialization by using recommonded sequence.

As ref_clk and ref_clk_src only required when using differential clock
source with QUSB PHY. Make those clocks as optional here.

Change-Id: Ic9a4d8c843b5d4318197f170a6d480954ae47ca9
Signed-off-by: Mayank Rana <mrana@codeaurora.org>