MIPS: csrc-r4k: Only use VDSO_CLOCKMODE_R4K when it is a available

MIPS: csrc-r4k: Only use VDSO_CLOCKMODE_R4K when it is a available

VDSO_CLOCKMODE_R4K is only defined if CONFIG_GENERIC_GETTIMEOFDAY is
enabled. Right now this is always the case, but it will change soon.

Prepare for the potential unavailability of VDSO_CLOCKMODE_R4K.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Link: https://patch.msgid.link/20260521-vdso-mips-kconfig-v1-5-2f79dcd6c78f@linutronix.de