arm: mvebu: theadorable: Add board-specific PEX detection pulse width

arm: mvebu: theadorable: Add board-specific PEX detection pulse width

Define a board-specific detection pulse-width array for the SerDes PCIe
interfaces. If not defined in the board code, the default of currently 2
is used. Values from 0...3 are possible (2 bits).

In this case of the theadorable board, PEX interface 0 needs a value
of 0 for the detection pulse width so that the PCIe device (Atheros
WLAN PCIe device) is consistantly detected.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Adam Shobash <adams@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Konstantin Porotchkin <kostap@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>