ARM: 8342/1: VDSO: depend on CPU_V7

When targeting ARMv3 (e.g. rpc) and enabling CONFIG_VDSO we get:

arch/arm/vdso/datapage.S:13: Error: selected processor does not
support ARM mode `bx lr'

One fix considered was to use 'ldr pc,lr' for such configurations, but
since the VDSO is unlikely to be useful for pre-v7 hardware, just make
it depend on CONFIG_CPU_V7.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nathan Lynch <nathan_lynch@mentor.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>