mips: Introduce vdso_direct

vdso_direct is an architectural clock configuration parameter that
exposes the usage of not of the vDSO.

If vdso_direct is false, the time related syscalls will be used instead
of the vDSO library.

This patch introduces the parameter in the mips architecture to prepare
the ground for the support of the generic vDSO library.

Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>