acpi: Allow finding the RSDT or XSDT

At present acpi_find_table() support locating most of the tables, but
not these two, since they are the pointers to the rest.

When adding new tables, these tables need to be located and updated, so
update acpi_find_table() to allow searching for them.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/dm/acpi.c

@@ -809,6 +809,9 @@ static int dm_test_acpi_find_table(struct unit_test_state *uts)
 	ut_asserteq_ptr(table3, table);
 	ut_asserteq_strn("TST3", table->signature);
 
+	table = acpi_find_table("RSDT");
+	ut_asserteq_ptr(nomap_sysmem(rsdt, 0), table);
+
 	/* Find with XSDT only */
 	rsdp->rsdt_address = 0;
 	acpi_udpate_rsdp_checksum(rsdp);