ArmPlatformPkg: Fix PL011 Glitches.

ArmPlatformPkg: Fix PL011 Glitches.

This change corrects 3 problems in the PL011 driver.
1. The TRM states "The UARTLCR_H, UARTIBRD, and UARTFBRD registers must
   not be changed:...when the UART is enabled"
2. The TRM (3.3.8) describes logic requiring the UART to be disabled and
   flushed before adjusting UARTCR.
3. Several redundant calls get made to PL011UartInitializePort, where
   the characteristics do not change, but updating the registers can
   cause glitches in the output stream.

The parameters are compared to the current state and no action taken if
no change of state is required.
Where an update is required, the specified logic is followed, and the
register updates only made when the UART is disabled.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Tested-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>