ram: rockchip: px30: add a config-based ddr selection

ram: rockchip: px30: add a config-based ddr selection

The SRAM on the PX30 is not big enough to hold multiple DDR configs
so it needs to be selected during build.

So far simply the DDR3 config was always selected and getting DDR4
or LPDDR2/3 initialized would require a code modification.

So add Kconfig options similar to RK3399 to allow selecting the DDR4
and LPDDR2/3 options instead, while DDR3 stays the default as before.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang<kever.yang@rock-chips.com>