[ARM] Orion: PCIe x4/x1 detection support

The Discovery Duo (MV78xx0) has two x4 PCIe ports which can either
be used in x4 mode or in quad x1 mode.  This patch adds an accessor
function to the generic plat-orion PCIe handling code to detect in
which of the two modes we're running (which is determined by strap
pins and/or configured by the bootloader).

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>