ArmPlatformPkg: reorganize PL011 code

ArmPlatformPkg: reorganize PL011 code

The PL011 code in ArmPlatformPkg is organized in a weird way: there is
a single PL011Uart.h header file under Include/Drivers containing both
register definitions and function entry points. The PL011Uart library
itself is in Drivers/ but it is actually a library.

So let's clean this up: add a new PL011UartLib library class and associated
header file containing only the library prototypes, and move the library
itself under Library/ using a new GUID, with the register definitions moved
into a local header file.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>