Move the PLL calculation code into it's own header
file for re-use with the other plat-s3c24xx based
systems such as the S3C24A0.

Note, we change the name of s3c2410_get_pll to the
more generically named s3c24xx_get_pll as well as
the related defintions.
Signed-off-by: Ben Dooks <ben-linux@fluff.org>