soc/mediatek/common/pcie: Add DEVTREE_CONST qualifier

Currently pcie.c is built into ramstage only, where DEVTREE_CONST is an
empty macro, so there's no problem with that. However, if we would like
to include that file in pre-ramstage, then DEVTREE_CONST would be
'const', leading to the following build error:

```
src/soc/mediatek/common/pcie.c:104:26: error: assignment discards
'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
104 |                 root_dev = pcidev_path_on_root(devfn);
    |                          ^
```

BUG=none
TEST=emerge-cherry coreboot

Change-Id: Ia7c95424019ec0dca50bbc6be7f81b6180d06d6e
Signed-off-by: Yidi Lin <yidilin@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84113
Reviewed-by: Jianjun Wang <jianjun.wang@mediatek.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>

src/soc/mediatek/common/pcie.c

@@ -96,7 +96,7 @@ static const char *const ltssm_str[] = {
 
 static uintptr_t mtk_pcie_get_controller_base(pci_devfn_t devfn)
 {
-	struct device *root_dev;
+	DEVTREE_CONST struct device *root_dev;
 	const mtk_soc_config_t *config;
 	static uintptr_t base = 0;