efi_loader: TPL_HIGH_LEVEL not allowed for CreateEvent

According to chapter 7.1 "Event, Timer, and Task Priority Services"
TPL_HIGH_LEVEL should not be exposed to applications and drivers.

According to the discussion with EDK II contributors this implies that
CreateEvent() shall not allow to create events with TPL_HIGH_LEVEL.

Cc: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Samer El-Haj-Mahmoud <Samer.El-Haj-Mahmoud@arm.com>