CLK: ti: dra7: Initialize USB_DPLL

USB_DPLL must be initialized and locked at boot so that
USB modules can work.

Also program USB_DLL_M2 output to half rate.

CC: Mike Turquette <mturquette@linaro.org>
CC: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>