[stretch] Enable CONFIG_HOTPLUG_PCI for arm64

Per 915231, this MR enables PCI device hotplug on arm64.

Output showing this functionality handling attach/detach of an NVMe block device on an Amazon EC2 arm64 instance:

admin@ip-172-31-11-63:~$ lspci
00:00.0 Host bridge: Amazon.com, Inc. Device 0200
00:01.0 Serial controller: Amazon.com, Inc. Device 8250
00:04.0 Non-Volatile memory controller: Amazon.com, Inc. Device 8061
00:05.0 Ethernet controller: Amazon.com, Inc. Elastic Network Adapter (ENA)

Kernel output when device is attached:

[ 2508.186361] pci 0000:00:1f.0: [1d0f:8061] type 00 class 0x010802
[ 2508.186419] pci 0000:00:1f.0: reg 0x10: [mem 0x00000000-0x00003fff]
[ 2508.186950] pci 0000:00:1f.0: PME# supported from D0 D1 D2 D3hot D3cold
[ 2508.187227] pci 0000:00:1f.0: BAR 0: assigned [mem 0x80118000-0x8011bfff]
[ 2508.190248] nvme nvme1: pci function 0000:00:1f.0
[ 2508.191561] nvme 0000:00:1f.0: enabling device (0000 -> 0002)

admin@ip-172-31-11-63:~$ lspci
00:00.0 Host bridge: Amazon.com, Inc. Device 0200
00:01.0 Serial controller: Amazon.com, Inc. Device 8250
00:04.0 Non-Volatile memory controller: Amazon.com, Inc. Device 8061
00:05.0 Ethernet controller: Amazon.com, Inc. Elastic Network Adapter (ENA)
00:1f.0 Non-Volatile memory controller: Amazon.com, Inc. Device 8061