arm: mvebu: theadorable: Add PEX-switch detection and reset code

arm: mvebu: theadorable: Add PEX-switch detection and reset code

Sometimes the PCIe link for the PEX-switch will not come-up. In this case,
the board is not in a usable state. This patch makes sure that in this
case a soft-reset is issued. If this soft-reset does not result in the
PEX-switch being detected after some soft-reset cycles, an I2C message
is sent to the uC to issue a complete power-cycle of the board.

Signed-off-by: Stefan Roese <sr@denx.de>