arm64: kconfig: move emulation option under kernel features

commit 1b907f46db07405b6676addb91b32c546d772fcd upstream.

Having the instruction emulation submenu underneath "platform selection"
is a great way to hide options we don't want people to use, but somewhat
confusing when you stumble across it there.

Move the menuconfig option underneath "kernel features", where it makes
a bit more sense.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: David Brown <david.brown@linaro.org>