[U-Boot] m53menlo: fix addmtd cmd in default environment

[sbabic at denx.de]

> The original definition added the string mtdparts= to the Linux Kernel
> args twice: mtdparts=mtdparts=. Fix that.
> Signed-off-by: Olaf Mandel <o.mandel at menlosystems.com>
> Reviewed-by: Marek Vasut <marex at denx.de>

Applied to u-boot-imx, master, thanks !

Best regards,
Stefano Babic

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