clocksource/drivers/versatile: Allow CONFIG_CLKSRC_VERSATILE to be disabled

The timer-versatile driver provides a sched_clock for certain Arm Ltd.
reference platforms. Specifically, it is used on Versatile and 32-bit
VExpress. It is not needed for those platforms with an arch timer (all
the 64-bit ones) yet CONFIG_MFD_VEXPRESS_SYSREG does still need to be
enabled. In that case, the timer-versatile can only be disabled when
COMPILE_TEST is enabled which is not desirable. Let's use the sub-arch
kconfig symbols instead.

Realview platforms don't have the sysregs that this driver uses so
correct the help text.

Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200417212045.16917-1-robh@kernel.org