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>
diff --git a/arch/powerpc/include/asm/fsl_serdes.h b/arch/powerpc/include/asm/fsl_serdes.h
index 1106d28..cce892c 100644
--- a/arch/powerpc/include/asm/fsl_serdes.h
+++ b/arch/powerpc/include/asm/fsl_serdes.h
@@ -86,6 +86,7 @@
 
 int is_serdes_configured(enum srds_prtcl device);
 void fsl_serdes_init(void);
+const char *serdes_clock_to_string(u32 clock);
 
 #ifdef CONFIG_FSL_CORENET
 #ifdef CONFIG_SYS_FSL_QORIQ_CHASSIS2