ARM: imx: pllv1: Fix PLL calculation for i.MX27

MFN bit 9 on i.MX27 has a different meaning than in other SOCs. This
is a just sign bit. This patch makes different calculation for i.MX27.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>