CLOCKSOURCE: versatile: Add PLAT_VERSATILE dependency

CLOCKSOURCE: versatile: Add PLAT_VERSATILE dependency

GENERIC_SCHED_CLOCK can be selected by architectures other than ARM. The
current dependencies of CLKSRC_VERSATILE make it possible that other
architectures will have CLKSRC_VERSATILE available in configuration once
they select GENERIC_SCHED_CLOCK, whereas this clock source should be solely
available to ARM in reality.

This patch adds one more dependency to CLKSRC_VERSATILE to fix the issue.

Signed-off-by: Deng-Cheng Zhu <dengcheng.zhu@imgtec.com>
Reported-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9476/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>