MIPS: Move r4k_wait() to .cpuidle.text section

Fix missing .cpuidle.text section assignment for r4k_wait() to correct
backtracing with nmi_backtrace().

Fixes: 97c8580e ("MIPS: Annotate cpu_wait implementations with __cpuidle")
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>