acpi_table: Support platforms with unusable RSDT

acpi_table: Support platforms with unusable RSDT

Since ACPI 2.0 the RSDT is deprecated and the XSDT should be preferred.
Until now the RSDT and XSDT entries were keept in sync as all platforms
that installed ACPI tables placed them below 4GiB and thus the address
would fit into the 32bit RSDT.

On platforms that do not have usable DRAM below 4GiB, like QEMU sbsa,
the RSDT cannot be used. Allow both RSDT and XSDT to be null and only
fill those tables that are present in acpi_add_table().

TEST: Fixes a crash on QEMU sbsa and allows to boot on QEMU sbsa.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>