arm64: defconfig: enable ARCH_NXP

Commit 566e373f ("arm64: Kconfig.platforms: Group NXP platforms
together") introduced a new symbol ARCH_NXP and made ARCH_LAYERSCAPE
(among others) depend on it, but didn't enable it in the defconfig.
Thus, now the defconfig doesn't include support for any NXP
architectures anymore. Fix it.

Fixes: 566e373f ("arm64: Kconfig.platforms: Group NXP platforms together")
Signed-off-by: Michael Walle <michael@walle.cc>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>