ARM: imx: imx8mn-evk: change environment address variables

Currently the space between kernel_addr_r and the fdt_addr_r is only 32MB.
To have enough space to load kernel images bigger than 32MB change the
variables to a feasible value.

The new environment variables layout is based on the scheme from
"include/configs/ti_armv7_common.h".

The CONFIG_SYS_LOAD_ADDR value is set to 0x42000000. With that we have
the same value as for the kernel_addr_r.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>