ARM: shmobile: Convert file to use cntvoff

Now that a common function is available for CNTVOFF's
initialization, let's convert shmobile-apmu code to use
this function.

Signed-off-by: Mylène Josserand <mylene.josserand@bootlin.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>