drivers: sh: pm_runtime does not need idle callback

In the runtime_pm idle callback the code assumes that a NULL .runtime_idle
entry is the same as a .runtime_idle entry that returns 0 as a result. This
means the entry in drivers/sh/pm_runtime can be removed in favour of just
leaving the entry NULL.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> [r8a7779 legacy]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>