configs: imx8mn_beacon: Re-align memory to standard imx8mn settings

The imx8mn_beacon board does not use the same memory map as the reference
design from NXP or other imx8mn boards.  As such, memory is more limited
in SPL.

Moving SPL_BSS_START_ADDR and SPL_STACK to default locations increases
the amount of available meory for the SPL stack.  Doing this allows
the board to no longer define CONFIG_MALLOC_F_ADDR.

Since SYS_LOAD_ADDR also does not align with other boards, move it too.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>