MLK-10006 PCI: imx6: pcie ep rc msi demo

- add one imx pcie ep simple skeleton driver to demo
the msi trigger capability in imx6 pcie rc/ep validation
system
- in order to avoid the modification of common codes,
force the msi address to be fixed.
(imx6sx:0x08ff8000, imx6q/dl:0x01ff8000)

Test howto on imx6sx:
(Replace the 08ff8000 by 01ff800 when imx6q/dl are used.)
- Enable CONFIG_PCI_MSI=y, when rebuild the rc/ep images

- EP side(console command and kernel message):
root@imx6sxsabresd:~# ./memtool -32 08ff8000=0
Writing 32-bit value 0x0 to address 0x08FF8000

- RC side(console command and kernel message):
root@imx6sxsabresd:~# cat /proc/interrupts  | grep MSI
384:          1   PCI-MSI

Signed-off-by: Richard Zhu <r65037@freescale.com>