ArmPlatformPkg: PL061 - only initialize on protocol load

For whatever reason, every single operation on the PL061 looked for an
"Initialized" flag, and manually called the initialization function if
not set. Move this to a single call on protocol installation.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>