kmp204x: initial support for PCIe FPGA configuration

The PEXHC PCIe configuration mechanism ensures that the FPGA get
configured at power-up. Since all the PCIe devices should be configured
when the kernel start, u-boot has to take care that the FPGA gets
configured also in other reset scenarios, mostly because of possible
configuration change.

The used mechanism is taken from the km_kirkwood design and adapted to
the kmp204x case (slightly different HW and PCIe configuration).

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Reviewed-by: York Sun <yorksun@freescale.com>