powerpc/fsl_soc: improve and simplify fsl_get_sys_freq

Use of_property_read_u32 instead of the generic of_get_property to
simplify the code. In addition move the declaration of sysfreq
into fsl_get_sys_freq because it's private to this function.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Scott Wood <oss@buserror.net>