mips: Add support for generic vDSO

The mips vDSO library requires some adaptations to use to take advantage
of the newly introduced generic vDSO library.

This patch introduces the following changes:
 - Modification of vdso.c to be compliant with the common vdso datapage
 - Use of lib/vdso for gettimeofday

Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Burton <paul.burton@mips.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>