[MIPS] time: Merge lasat plat_timer_setup into plat_time_init.

Since the cp0 compare interrupt handler isn't initialized by the time
plat_time_init is called don't set IE_IRQ5 anymore, cevt-r4k.c will do
that a little later itself.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>