rockchip: puma-rk3399: allow loading environment from SPI-NOR flash

There's a SPI-NOR flash available from which SPL and U-Boot proper can
be booted, it makes sense to also allow this medium to store U-Boot
environment so let's enable it.

The Device Tree advertises a max frequency of 50MHz so let's set the
config option appropriately.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>