coreboot_tables.c: Expose the ACPI RSDP

The ACPI RSDP can only be found in:
- legacy BIOS region
- via UEFI service

On some systems like ARM that legacy BIOS region is not an option, so
to avoid needing UEFI it makes sense to expose the RSDP via a coreboot
table entry.

This also adds the respective unit test.

Change-Id: I591312a2c48f0cbbb03b2787e4b365e9c932afff
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62573
Reviewed-by: Lance Zhao
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/acpi/acpi.c

@@ -1582,6 +1582,13 @@ void preload_acpi_dsdt(void)
 	cbfs_preload(file);
 }
 
+static uintptr_t coreboot_rsdp;
+
+uintptr_t get_coreboot_rsdp(void)
+{
+	return coreboot_rsdp;
+}
+
 unsigned long write_acpi_tables(unsigned long start)
 {
 	unsigned long current;
@@ -1689,6 +1696,7 @@ unsigned long write_acpi_tables(unsigned long start)
 
 	/* We need at least an RSDP and an RSDT Table */
 	rsdp = (acpi_rsdp_t *) current;
+	coreboot_rsdp = (uintptr_t)rsdp;
 	current += sizeof(acpi_rsdp_t);
 	current = acpi_align_current(current);
 	rsdt = (acpi_rsdt_t *) current;