rockchip: rk3399-puma: reduce env size to 8kiB

This commit changes the size of the enviroment (for the RK3399-Q7) to
8kiB for all possible locations of the environment (i.e. even when the
environment is saved to SD card).

With the default of 32kiB, the environment overwrites the SPL
stage which lives at 16kiB.

Signed-off-by: Jakob Unterwurzacher <jakob.unterwurzacher@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
[Reworked commit-message:]
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>