ram: rk3399: Rename sys_reg with sys_reg2

ram: rk3399: Rename sys_reg with sys_reg2

Use dram config variable name as sys_reg2 instead of sys_reg
since the final variable value is to written into a pmugrf
register named as sys_reg2.

This reflect the both variable and associated register
names are same and also help to add next sys_reg's to
add it in future.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: YouMin Chen <cym@rock-chips.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>