dt-bindings: pcie: add entry to specify max link training attempts

By default, PCIe device has 100ms to link up. Some devices
require more time so provide a devicetree entry to override
the number of link train attempts.

Change-Id: I7a0ecdf08732de1d0c28fda7ed8eae92d4df2076
Signed-off-by: Tony Truong <truong@codeaurora.org>