tests/pm_rps: wait longer for idling

tests/pm_rps: wait longer for idling

Big cores seem to take forever to idle, at least my ivb here. Fairly
ridiculous, so maybe there's more room for our debooster to kick in.

Anyway, this gets the min-max-config-idle and reset subtests going
somewhat reliably on my ivb. They still occasionally fail with the
current frequency pegged to max (or close to it) for no apparent
reason at all. Rebooting tends to fix it.

Don't ask, don't tell.

Cc: Jeff McGee <jeff.mcgee@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>