drivers: console: pl011: Move comments to header

Rather than having the description of each function in the
implementation, move it to the header so that it isn't needed to copy
it in each file that implements a console driver.

Change-Id: I1b437ae3d3ccff10979466727353bc1fa1d5b188
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>