arm64/rk_pcie_phy: handle assigned-clock*

Nanopi4 based SoCs (NanoPC-T4, NanoPi M4*, and NanoPi Neo4) have
assigned-clock* in the pcie_phy node. Handle them but only fail
in case clk_set_assigned() returns an error other than
"no assigned-clock*" (as it would for all other SoCs).

Reviewed by: manu
MFC After: 2 weeks
Differential Revision: https://reviews.freebsd.org/D30363