imx8mm_evk: Add Serial Download Protocol support

imx8mm_evk: Add Serial Download Protocol support

Add Serial Download Protocol support as it is a useful method to
load flash.bin to RAM and run it via 'uuu'.

With this patch, it is possible to start both U-Boot SPL and U-Boot
proper using the following 'uuu'command:

$ uuu -brun spl flash.bin

Based on a patch from Marek Vasut for the imx8mm-mx8menlo board.

Also, to fit the SPL binary into the internal RAM, select CONFIG_LTO
to reduce its size.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>