Added Sleep, LED3 and LED4 definitions, switches

Sleep = KL25Z sleep (verified to work correctly)
LED3 = LED1, LED4 = LED2 (for compatibility, similar to LED3 = LED4 in
KL25Z code)
SW1 and SW3 for the two switches (SW2 is reset switch)
Fixed KL25Z SPI clock

https://mbed.org/questions/2309/spifrequency-for-KL25Z/
SPI clock fix for KL05Z and KL46Z
Removed hardcoded frequencies, replaced by automatically calculated

ADC frequency 1.5MHz -> 6MHz (if possible, otherwise it tries as close
to, but less than 6MHz)
pwm tries to get its clock as close as possible to 1MHz, but not lower
than 1MHz
Updat PWM prescaler correctly
Added clk_freqs.h for clock related functions

Serial now also works when PLL/FLL is disabled and extosc is available.

us_ticker tries to divide extosc to 1MHz, otherwise use fast internal
oscillator
Merge pull request #140 from Sissors/master

KL46Z: Added Sleep, LED3 and LED4 definitions, switches