dt-bindings: pcie: add entry for PCIe PHY status bit

The bit for PCIe PHY status varies for each PCIe core.
Therefore, add a DT entry to specify which bit to should
be checked for PCIe PHY status.

Change-Id: If40322d2c131681608a627c51b5130e04f5a0320
Signed-off-by: Tony Truong <truong@codeaurora.org>