cmd: Kconfig: Fix a dependency of CMD_MTDPARTS

Remove the ARCH_SUNXI dependency.

Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Jagan Teki <jagan@openedev.com>
Signed-off-by: Karl Beldan <karl.beldan-ext@sagemcom.com>
Reviewed-by: Tom Rini <trini@konsulko.com>