add 256MHz

utility/KinetisSDHC.c

 #define SDHC_SYSCTL_DIVISOR(prescale, divisor) \
 	(SDHC_SYSCTL_SDCLKFS((prescale)>>1)|SDHC_SYSCTL_DVS((divisor)-1))
 
-#if (F_CPU == 240000000)
+#if (F_CPU == 256000000)
+  #define SDHC_SYSCTL_400KHZ  SDHC_SYSCTL_DIVISOR(64, 10) // 400 kHz
+  #define SDHC_SYSCTL_25MHZ   SDHC_SYSCTL_DIVISOR(2, 6)   // 21.3 MHz
+  #define SDHC_SYSCTL_50MHZ   SDHC_SYSCTL_DIVISOR(2, 3)   // 42.6 MHz
+#elif (F_CPU == 240000000)
   #define SDHC_SYSCTL_400KHZ  SDHC_SYSCTL_DIVISOR(64, 10) // 375 kHz
   #define SDHC_SYSCTL_25MHZ   SDHC_SYSCTL_DIVISOR(2, 5)   // 24 MHz
   #define SDHC_SYSCTL_50MHZ   SDHC_SYSCTL_DIVISOR(2, 3)   // 40 MHz