PCI: Add T_PERST_CLK_US macro

PCI: Add T_PERST_CLK_US macro

The "Power Sequencing and Reset Signal Timings" table of the PCI
Express Card Electromechanical Specification, Revision 5.1, Section
2.9.2, indicates PERST# should be deasserted after minimum of 100us
once REFCLK is stable (symbol T_PERST-CLK).

Add a macro so that PCIe controller drivers can use it.

Link: https://lore.kernel.org/linux-pci/[email protected]
Signed-off-by: Thomas Richard <[email protected]>
[kwilczynski: commit log, update sleep interval macros code comments]
Signed-off-by: Krzysztof Wilczyński <[email protected]>