Changed needed to support pin 45 on C3.5/6 as it is CS5 which encodes as 0x20...

As per liljaywalker member

Teensy 3.5/6 on Pin 45 have PCS5,
which the current Macro
SPI-PUSHR_PCS(n) will not work with.

Add VREF_SC_MODE_LV constants

Fixes #239

* add SMC_STOPCTRL

https://forum.pjrc.com/threads/43362-SMC_VLLSCTRL-register-is-not-found-in-K66-(Teensy-3-6)

* Update kinetis.h

Easy field manipulation for RTC_IER register

Different version applied upstream.

This reverts commit c5bdda910b.

Needed to add new Define for clock gate

Plus defines to use the new clock gate, plus correct case name for the
serial6 isr to match what is in system isr table

The T-LC and the new beta boards T3.6 and soon T3.5 hardware uarts have
ability to turn on 2 stop bits.  So enabled on these boards.

This is a WIP,  But I have now been able to create Serial6 on T3.4 beta,
and so far have tested Send/Receive basic stuff at 115200 and have tried
at several CPU speeds in MHZ (192, 216, 180, 120, 96)

Just in case the extra pads are added to bottom of 3.4/3.5 board,
define is in place...

Added define for SPI1 registers