rk3568 pcie: Do not require optional FDT properties.
- "vpcie3v3-supply" is documented as an optional, do not require it.
- fix handling of the another optional property "reset-gpios".
- while I'm in, fix errors reporting in rk3568_pcie_init_soc()
MFC after: 2 weeks
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D50102