rockchip: ARM64: puma-rk3399: get DRAM size from DMC init

rockchip: ARM64: puma-rk3399: get DRAM size from DMC init

With the RK3399 DRAM controller (DMC) driver providing all the
infrastructure, retrieve the DRAM size from the DMC init in the
board-specific code (instead of hard-coding) for the RK3399-Q7 (Puma).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>