BACKPORT: arm64: Introduce asm/vdso/clocksource.h

The vDSO library should only include the necessary headers required for
a userspace library (UAPI and a minimal set of kernel headers). To make
this possible it is necessary to isolate from the kernel headers the
common parts that are strictly necessary to build the library.

Introduce asm/vdso/clocksource.h to contain all the arm64 specific
functions that are suitable for vDSO inclusion.

This header will be required by a future patch that will generalize
vdso/clocksource.h.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lkml.kernel.org/r/20200320145351.32292-7-vincenzo.frascino@arm.com


(cherry picked from commit 31fdcac07f679848b5ba371500292b685fd39217)
Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 154668398
Change-Id: I14a1ecd16f960d7b70388ee3587dc5abe63e2f6a