Update usb.c

To allow future MTP_SERIAL implementations and facilitate use of MTP_t4, that provides possibility of MTP_SERIAL

Fix dumb mistake on my part - thanksKurt

Update usb.c so MTP_repsonder can work at 600Mhz

Update DMAChannel.h

Allow in both KINETISK and KINETISL DMA interrupt with priority
previous PR was only on one of them

T4.x Create Struct for IMXRT_GPIO

Wondered  if this had already been done earlier.

Was not so am trying it now.

One side effect was had to change interrupt.c as there were
#defines for things like DR, GDIR which then caused compile issues with the structure.  So renamed the defines.

Built about 3 programs with it so far.  Which appear to still be working.

Fixes to CSI registers

Missed a few, pllus one register fields name had wrong register name

imxrt.h - Add CSI Register field names

https://forum.pjrc.com/threads/63842

Update DMAChannel.h