rockchip: dts: rk3399-puma: put environment (in MMC/SD configurations) before SPL

As our SPL stage can grow quite large (80KB+ are not unusual) on the
RK3399-Q7, the default setting for the environment location (in
include/configs/rockchip-common.h) can overlap our SPL.

This change finally makes use of the 'u-boot,mmc-env-offset' DTS
property to override the environment location and put it at 16KB into
the device, which is right before the SPL (located at 32KB).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>