ARM: vPL011: Use the VGIC's level triggered IRQs handling if available

ARM: vPL011: Use the VGIC's level triggered IRQs handling if available

The emulated ARM SBSA UART is using level triggered IRQ semantics,
however the current VGIC can only handle edge triggered IRQs, really.
Disable the existing workaround for this problem in case we have the
new VGIC in place, which can properly handle level triggered IRQs.

Signed-off-by: Andre Przywara <andre.przywara@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>