arm: dts: imx8mp: Generate single bootable binary

arm: dts: imx8mp: Generate single bootable binary

binman conversion made flashing flash.bin
and u-boot.itb necessary. Update binman config
to create a single flash.bin image again.

This updates imx8mp_evk and phyCORE-i.MX8MP as they share the
same binman config.

Updated also imx8mp_evk documentation.

Tested on phyCORE-i.MX8MP.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>