fsl/mpc85xx: define common serdes_clock_to_string function

This allows to share some common code for the boards that use a corenet
base SoC.

Two different versions of the function are available in
fsl_corenet_serdes.c and fsl_corenet2_serdes.c files.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
[York Sun: fix t1040qds.c]
Acked-by: York Sun <yorksun@freescale.com>