ArmPlatformPkg: Store initial timer value

ArmPlatformPkg: Store initial timer value

This commit adds support for storing initial timer value
logged at the beginning of firmware image execution.
This timer value is required for firmware basic boot performance
data record referenced by Firmware Performance Data Table (FPDT).

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Alexei Fedorov <Alxei.Fedorov@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Reviewed-by: Graeme Gregory <graeme.gregory@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>