soc/intel/xeon_sp/ebg/soc_xhci: Check if BAR is reachable

On x86_32 the xHCI BAR isn't reachable as it's mapped in high MMIO.
Currently this is not a problem since the code is unused.

Add a check and return NULL instead of cutting of the higher bits
and thus do not return an invalid pointer. On x86_64 it's working
when the extended page-tables are installed.

Change-Id: I00496ad476c33e0984d7cb0019f27154302edda5
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/85809
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Shuo Liu <shuo.liu@intel.com>

src/soc/intel/xeon_sp/ebg/soc_xhci.c

@@ -17,6 +17,13 @@ static uint8_t *get_xhci_bar(void)
 		printk(BIOS_ERR, "XHCI BAR is not found\n");
 		return NULL;
 	}
+
+#if ENV_X86_32
+	assert(res->base < 0x100000000ULL);
+	if (res->base >= 0x100000000ULL)
+		return NULL;
+#endif
+
 	return (void *)(uintptr_t)res->base;
 }