ipq806x: fix freeze in PCIe code when booting with an old u-boot

ipq806x: fix freeze in PCIe code when booting with an old u-boot

Old bootloader (same ones which have DT disabled) don't perform any PCIe
initialization. The consequence is a freeze during PCIe bring-up on
these old u-boot. Same kernel with a newer bootloaders works fine as
they contain the corresponding PCIe init code.

In this change, we'll add the missing init and make sure the kernel
doesn't rely on some preexisting init to get PCIe to work. That includes
the following changes:
*GPIOs: set function & drive strength
*Clocks: add init code for aux & ref clocks
*PCIe driver: additional init of the hardware controller

Tested 3.18 and 4.1 on an AP148 with bootloader branch 0.0.1

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46557 3c298f89-4303-0410-b956-a3cf2f4a3e73