Fixed KL25Z SPI clock

https://mbed.org/questions/2309/spifrequency-for-KL25Z/

Author: Sissors

libraries/mbed/targets/hal/TARGET_Freescale/TARGET_KL25Z/spi_api.c

@@ -145,7 +145,7 @@ void spi_frequency(spi_t *obj, int hz) {
     uint8_t  ref_prescaler = 0;
 
     // bus clk
-    uint32_t PCLK = 48000000u;
+    uint32_t PCLK = SystemCoreClock / (((SIM->CLKDIV1 & SIM_CLKDIV1_OUTDIV4_MASK) >> SIM_CLKDIV1_OUTDIV4_SHIFT) + 1);
     uint8_t prescaler = 1;
     uint8_t divisor = 2;