arm64: kconfig: move emulation option under kernel features

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.

Change-Id: Ib31f17503ec4a6e79c3c0c0497ac699d8e978dab
Signed-off-by: Will Deacon <will.deacon@arm.com>