ubi: Kconfig: Fix MTD_UBI selection dependency

Remove the ARCH_SUNXI dependency on selection of RBTREE.

Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
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>