ARM: orion: avoid VLA in orion_mpp_conf

Testing randconfig builds found an instance of a VLA that was
missed when determining that we have removed them all:

arch/arm/plat-orion/mpp.c: In function 'orion_mpp_conf':
arch/arm/plat-orion/mpp.c:31:2: error: ISO C90 forbids variable length array 'mpp_ctrl' [-Werror=vla]

This one is fairly straightforward: we know what all three
callers are, and the maximum length is not very long.

Fixes: 68664695ae57 ("Makefile: Globally enable VLA warning")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Olof Johansson <olof@lixom.net>