arm64: cpufeature: Move cpu capability helpers inside C file

These helpers are used only by functions inside cpufeature.c and
hence makes sense to be moved from cpufeature.h to cpufeature.c as
they are not expected to be used globaly.

This change helps in reducing the header file size as well as to add
future cpu capability types without confusion. Only a cpu capability
type macro is sufficient to expose those capabilities globally.

Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>