MTD_XIP depends on having working asm/mtd-xip.h; it's not just per-architecture
(arm-only, as current Kconfig would have it), but actually per-subarch as
well.  Introduced a new symbol (ARCH_MTD_XIP) set by arch Kconfig; MTD_XIP
depends on it.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>