arm64: move {PAGE,CONT}_SHIFT into Kconfig

In some cases (e.g. the awk for CONFIG_RANDOMIZE_TEXT_OFFSET) we would
like to make use of PAGE_SHIFT outside of code that can include the
usual header files.

Add a new CONFIG_ARM64_PAGE_SHIFT for this, likewise with
ARM64_CONT_SHIFT for consistency.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>