arm: vdso: Fix GENERIC_GETTIMEOFDAY dependencies

GENERIC_GETTIMEOFDAY can be enabled only if HAVE_GENERIC_VDSO
is enabled. However, HAVE_ARM_ARCH_TIMER selects it unconditionally,
thus throwing the following warning:

WARNING: unmet direct dependencies detected for GENERIC_GETTIMEOFDAY
Depends on [n]: HAVE_GENERIC_VDSO [=n]
Selected by [y]:
- HAVE_ARM_ARCH_TIMER [=y] && CPU_V7 [=y]

Fix this by selecting GENERIC_GETTIMEOFDAY conditionally in
HAVE_ARM_ARCH_TIMER as well.

Also, handle this conditional dependency in all the references.

Change-Id: Iccb154ad4c0f49857115372334e49504f31a8380
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>