Platform/RPi4: Add ACPI basic mode build option

Add an ACPI_BASIC_MODE_ENABLE flag to produces builds intended to run in
ACPI mode without any additional requirements (memory limits, acpi=force,
etc).

This flag is disabled by default.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>