platsupport: TK1 UART: Add comments and memory barrier

* Minor suggestion for the future to add PAUSE or some other opcode to a busy loop
* Add a memory barrier where one should exist.

platsupport: TK1 UART: Replace dispersed callbacks with invoke_callback

platsupport: TK1 UART: Consolidate callback invocation

The client should be able to indicate that it wishes to shut off IRQs when by setting
bytes_requested to 0. This is an alternative to adding an explicit API call to terminate
ongoing asynchronous xfers.

* Implement tk1_uart_invoke_callback().

This function groups all the callback processing into one place to reduce
duplication.

* Callbacks should only be generated if a client has issued a read/write call to begin
with.
* Disable async IRQs if bytes_requested is set to 0

platsupport: TK1 UART: Fix baud calculation bug

* Ever so slightly change the algorithm used to compute the divisor for the
UARTs.

Fix whitespace

- remove trailing whitespace
- remove duplicate blank lines
- remove blank lines at end of file

fix licenses

NV/TK1: Serial: Increase TX FIFO size to fix newline issue

TK1: Fix UART parity generation

The existing code works only for NO PARITY. Fix this; and add
PARITY_ZERO and PARITY_ONE example code.

libplatsupport: add tx1 platform

libplatsupport: create mach folder for nvidia machines