ARM: shmobile: R-Car: Break infinite loop

rcar_sysc_update() loops (with interrupts disabled and while holding a
spinlock) until submitting a power shutoff or resume request fails, or
until the submitted request was accepted.
If none of these conditions becomes true, this forms an infinite loop.

Put a limit on the maximum number of loop iterations, and add a small
delay to each iteration, to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>