soc/mediatek/mt8173: Allow BL31 payload not targeting RAM

selfboot.c blocks the payload that does not target RAM. But MT8173 loads
and runs BL31 payload in SRAM. Make the exception by implementing
`payload_arch_usable_ram_quirk()`.

TEST=load and initialize BL31 successfully

Change-Id: I8951b1c4673cdae7d1ad0c11d7d6c12376acd328
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70344
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/mediatek/mt8173/soc.c

@@ -2,9 +2,23 @@
 
 #include <bootmem.h>
 #include <device/device.h>
+#include <program_loading.h>
 #include <symbols.h>
 #include <soc/emi.h>
 
+int payload_arch_usable_ram_quirk(uint64_t start, uint64_t size)
+{
+	if (size > REGION_SIZE(sram))
+		return 0;
+
+	if (start >= (uintptr_t)_sram && (start + size) <= (uintptr_t)_esram) {
+		printk(BIOS_DEBUG, "MT8173 uses SRAM for loading BL31.\n");
+		return 1;
+	}
+
+	return 0;
+}
+
 void bootmem_platform_add_ranges(void)
 {
 	bootmem_add_range(0x101000, 124 * KiB, BM_MEM_BL31);