arm64: add a std.ec2 config

Mostly to document basic harware present on the platform; knowing that
Graviton exposes an ns8250 uart alone is quite helpful.

Reviewed by:	andrew, imp, manu
Seems accurate:	cperciva
Sponsored by:	NetApp, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D36776

sys/arm64/conf/GENERIC

@@ -31,6 +31,7 @@ include		"std.amd"
 include		"std.arm"
 include		"std.broadcom"
 include		"std.cavium"
+include		"std.ec2"
 include		"std.hyperv"
 include		"std.hisilicon"
 include		"std.imx"

sys/arm64/conf/std.ec2

@@ -0,0 +1,15 @@
+#
+# EC2 support (Graviton, Graviton 2)
+#
+
+# Serial (COM) ports
+device		uart_ns8250
+
+# Ethernet NIC, typically loaded as a module instead
+#device		ena
+
+# Options typically present in AWS /boot/loader.conf that we don't want to
+# enable for GENERIC
+#envvar		hw.broken_txfifo=1
+
+device		acpi