doc: imx8mp_evk: Remove unneeded export ATF_LOAD_ADDR line

Originally, exporting the ATF_LOAD_ADDR was required, but since binman has
been used to generate the flash.bin, it is no longer needed to do
such manual export.

The ATF address is now passed via binman in imx8mp-u-boot.dtsi:

	atf {
		description = "ARM Trusted Firmware";
		type = "firmware";
		arch = "arm64";
		compression = "none";
		load = <0x970000>;
		entry = <0x970000>;

		atf_blob: atf-blob {
			filename = "bl31.bin";
			type = "atf-bl31";
		};
	};

Remove the  unneeded export ATF_LOAD_ADDR line.

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