ram: k3-ddrss: Allow use of dt provided initial frequency

Allow device tree to provide ti,ddr-freq0 to be used as the initial DDR
frequency that is set for lpddr4 before initialization of the
controller. Make this optional and continue to use PLL bypass frequency
as is done currently if ti,ddr-freq0 is not provided.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>