ArmPkg: Tidy GIC code before changes.

ArmPkg: Tidy GIC code before changes.

This change is purely cosmetic, to tidy some code before change.
Mods involve:
    Re-order #includes
    Reformat comments.
    Use ns consistently (always "100ns" not sometimes "100 nS")
    Split overlength code lines.
    Make protocol functions STATIC.
    Remove "Horor vacui" comments.
    Rationalize GIC register address calculations
    Replace explicit test and assert with ASSERT_EFI_ERROR.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Girish Pathak <girish.pathak@arm.com>
Signed-off-by: Alexei Fedorov <alexei.fedorov@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>