[PATCH] imx8mp_evk: README instruction fixes

[sbabic at denx.de]

> Use the full name of firmware self extracting file to make it run.
> Also, don't use sudo when not needed.
> Signed-off-by: Baruch Siach <baruch at tkos.co.il>
Applied to u-boot-imx, master, thanks !

Best regards,
Stefano Babic

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