When programming the inbound/outbound ATUs, we call usleep_range() after
each checking PCIE_ATU_ENABLE bit. Unfortunately, the ATU programming
can be executed in atomic context:
inbound ATU programming could be called through
pci_epc_write_header()
=>dw_pcie_ep_write_header()
=>dw_pcie_prog_inbound_atu()
outbound ATU programming could be called through
pci_bus_read_config_dword()
=>dw_pcie_rd_conf()
=>dw_pcie_prog_outbound_atu()