arm: mvebu: AXP: Add possiblity to configure PEX detection pulse width

Tests have shown that on some boards the default width of the
configuration pulse for the PEX link detection might lead to
non-established PCIe links (link down). Especially under certain
conditions (higher temperature) and with specific PCIe devices
(in the case on the theadorable board its a Atheros PCIe WLAN
device). To enable a board-specific detection pulse width this weak
array "serdes_pex_pulse_width[4]" is introduced which can be
overwritten if needed by a board-specific version. If the board
code does not provide a non-weak version of this variable, the
default value will be used. So nothing is changed from the
current setup on the supported board.

Many thanks to Adam from Marvell for all his insights here and
his suggestion about testing with a changed detection pulse width.

Signed-off-by: Stefan Roese <sr@denx.de>
Suggested-by: Adam Shobash <adams@marvell.com>
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>