arm64: enable time namespace support

CONFIG_TIME_NS is dependes on GENERIC_VDSO_TIME_NS.

Signed-off-by: Andrei Vagin <avagin@gmail.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Dmitry Safonov <dima@arista.com>
Link: https://lore.kernel.org/r/20200624083321.144975-7-avagin@gmail.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>