[ARM] 3990/1: i.MX/MX1 more precise PLL decode

The future high resolution support inclusion utilizes
imx_decode_pll() in timer base frequency computation.
This use requires more precise computation without
discarding 10 bits by shifting left.

Signed-off-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>