clock-imx35: fix divider if ahb is source for ipg_per

According to the Datasheet:
"i.MX35 (MCIMX35) Multimedia Applications Processor Reference Manual,
Rev. 2" "Table 14-6. PDR0 Field Descriptions" the divider is
CCM_PER_AHB[3:0] + 1.

This patch adds the missing + 1.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>