the dram init is board related. But there is still some common
part can be reused on different board. The basic flow is common
for all the board. only the DDRC and DDR PHY config register setting
is different on different board. So extract the LPDDR4 init common
flow to make it more generic. baord level only need to provide
the DDRC and PHY config register parameter to the common code to finish
the dram init.

the same method can be use for DDR4. will be added later.

Signed-off-by: Bai Ping <ping.bai@nxp.com>