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

An experiement to see how much work again it would take to add the T4 methods to allow user sketches to increase the size of Serial ports RX and TX buffers, without having to edit the Source code which does it then for all sketches.

T3.x SerialN Add extra memory capabilities for Serial1-6

Also

Fixed serialEventX yield handling when you call begin multiple times

There is an issue that if you are not using the serialEvent type code and you call SerialX.begin multiple times, it will add that item to the process list multiple times, which can overrun the memory and cause crash or the like.

Fix was to check the list first and see if we are already in the list.

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