serial: xuartps: Rewrite the interrupt handling logic

The existing interrupt handling logic has followins issues.
- Upon a parity error with default configuration, the control
  never comes out of the ISR thereby hanging Linux.
- The error handling logic around framing and parity error are buggy.
  There are chances that the errors will never be captured.
- The existing ISR is just too long.
This patch fixes all these concerns. It separates out the Tx and Rx
hanling logic into separate functions. It ensures that the status
registers are cleared on all cases so that a hang situation never
arises.

Signed-off-by: Anirudha Sarangi <anirudh@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>