ath79: add QCA9550 reset sequence

ath79: add QCA9550 reset sequence

The QCA9550 family of SoCs have a slightly different reset
sequence compared to older chips.

Normally the bootloader performs this sequence, however
some bootloader implementation expect the operating system
to clear the reset. Also get the PCIe resets from OF to
support the second RC of the QCA9558.

This is required for the AVM FRITZ!WLAN Repeater 1750E to work,
as EVA leaves the PCIe bus in reset.

Tested: AVM FRITZ!WLAN Repeater 1750E - OCEDO Koala

Signed-off-by: David Bauer <mail@david-bauer.net>