arm: mvebu: turris_omnia: Provide env_sf_get_env_addr() function

BootROM maps SPI Flash to fixed address 0xD4000000 and this mapping is
active also when BootROM is executing binary kwbimage headers, which
includes also U-Boot SPL.

Therefore no initialization code is required to access SPI Flags from
U-Boot SPL. In proper U-Boot it is remapped to other location.

So in mvebu implementation of env_sf_get_env_addr() function returns
0xD4000000 when running in SPL and NULL when in proper U-Boot.

This change would allow to use U-Boot ENV in U-Boot SPL. Normally it is not
possible to read ENV because it is too big and U-Boot SPL does not have
such big malloc() pool to real all ENV variables.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>