ar71xx: mark soft_config mtd part as writeable for RB-941-2nD
The soft_config partition must be writeable for rbcfg to be able to
enact changes to the routerboot configuration.
The read-only flag was a mistake in the initial patch. Removing it
brings mach-rb941.c in line with all other RB platforms.
Signed-off-by: Thibaut VARENE <[email protected]>