Setup CTS, DSR, DCD, RI interrupts

Setup EXTI sources, set trigger direction and added stub interrupt
handlers for CTS, DSR, DCD, RI inputs.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Fix up interrupt priorities, ignoring bits[3:0]

Bits[3:0] of the 8 bit interrupt priority are ignored so shift interrupt
priorities by 4 bits.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Send notifications on CTS,DSR,DCD,RI changes

Notifications are buffered for up to approximately 32 character times.
When data received from the UART is sent to the USB host the state of
CTS,DSR,DCD,RI is also included.

Transmit empty notifications are delayed for approximately 1 to 2
character times or a minimum of 1ms.

Signed-off-by: George McCollister <george.mccollister@gmail.com>
Added hardware flow control support

Added non-standard USB_CDC_SET_FLOW_CONTROL which I use in the linux
driver to enable/disable hardware flow control.

Signed-off-by: George McCollister <george.mccollister@gmail.com>