ARM: 8987/1: VDSO: Fix incorrect clock_gettime64

ARM: 8987/1: VDSO: Fix incorrect clock_gettime64

__vdso_*() should be removed and fallback used if CNTCVT is not
available by cntvct_functional(). __vdso_clock_gettime64 when added
previous commit is using the incorrect CNTCVT value in that state.
__vdso_clock_gettime64 is also added to remove it's symbol.

Cc: stable@vger.kernel.org
Fixes: b1c9dc549e00 ("ARM: 8932/1: Add clock_gettime64 entry point")
Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Tested-by: Robin Murphy <robin.mruphy@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>