clk: vexpress: Make the clock drivers directly available for arm64

The new arm64 architecture has no idea of platform or machine, so
it doesn't have to define ARCH_VEXPRESS configuration option at
all. To allow user to select the drivers at all, make it depend
on ARM64 as well.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>