intel/common/block/pcie: Add NULL/count check in pcie_rp_update_devtree

pcie_rp_update_devicetree function takes pcie_rp_group strcuture
as an argument and SoC code passes the parameter in this structure.

This pointer can be NULL and common code may try to dereference
this NULL pointer.

Also, group might have no data and SoC may pass this by indicating
group count as zero (For example, for CPU or TBT root ports).

These checks will prevent function from executing redundant code
and returning early from the call as it's not required.

BUG=b:210933428
BRANCH=None
TEST=check if function returns early for group count 0 and there is
no issue while booting board in case group count = 0.

Change-Id: I132319bb05fdef1590c18302fc64cc76e15bea6d
Signed-off-by: MAULIK V VAGHELA <maulik.v.vaghela@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61331
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: EricR Lai <ericr_lai@compal.corp-partner.google.com>
Reviewed-by: Subrata Banik <subratabanik@google.com>

src/soc/intel/common/block/pcie/pcie_rp.c

@@ -130,6 +130,9 @@ void pcie_rp_update_devicetree(const struct pcie_rp_group *const groups)
 	int mapping[CONFIG_MAX_ROOT_PORTS];
 	unsigned int offset, i;
 
+	if (!groups || !groups->count)
+		return;
+
 	struct bus *const root = pci_root_bus();
 	if (!root)
 		return;