arm64: Kconfig.platforms: Group NXP platforms together

Group the three NXP platforms under an ARCH_NXP menuconfig symbol to
make make selection of similar vendor SoCs visually nicer.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220829173830.3567047-3-f.fainelli@gmail.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>