soc/mediatek: Don't attempt de-assert PERST# without pci_root_bus

Since CB:84118 / 3d5ff65b27 (mb/google/cherry: Complete PCIe reset in
romstage) google-cherry mainboards do an early PERST# de-assert in
romstage. Since cherry does not have a pci_domain, `pci_root_bus()` will
return null, causing an assertion failure later in `find_dev_path()`.

Return if `pci_root_bus()` is NULL.

TEST=Successful boot on google/tomato

Change-Id: Icc35a53e38eef0088371592d8216ac74f9542166
Signed-off-by: Ingo Reitz <9l@9lo.re>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/88035
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yidi Lin <yidilin@google.com>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>

src/soc/mediatek/common/pcie.c

@@ -333,6 +333,9 @@ static void wait_perst_done(uintptr_t base)
 
 void mtk_pcie_deassert_perst(void)
 {
+	if (!pci_root_bus())
+		return;
+
 	uintptr_t base = mtk_pcie_get_controller_base(0);
 
 	wait_perst_done(base);