arm64: start defining a std.apple configuration

The M1 uses FDT, and has bge to start with.  Add a SOC_* option for
the first SoC we'll be supporting.

IOMMU is added commented out because it does have it, but IOMMU is not
well-tested on aarch64.  An initial version of the DART driver will be
upstreamed that just puts the DARTs that support bypass mode into bypass
mode -- we'll be missing some functionality, but we at least still end
up with some USB ports.

Reviewed by:	karels, manu
Input from:	jrtc27 (IOMMU)
Differential Revision:	https://reviews.freebsd.org/D39823

sys/arm64/conf/GENERIC

@@ -28,6 +28,7 @@ include		"std.al"
 include		"std.allwinner"
 include		"std.altera"
 include		"std.amd"
+include		"std.apple"
 include		"std.arm"
 include		"std.azure"
 include		"std.broadcom"

sys/arm64/conf/std.apple

@@ -0,0 +1,10 @@
+#
+# Apple SoC support
+#
+
+options 	SOC_APPLE_T8103
+
+#options 	IOMMU
+options 	FDT
+
+device	bge

sys/conf/options.arm64

@@ -23,6 +23,7 @@ EFIRT				opt_efirt.h
 SOC_ALLWINNER_A64		opt_soc.h
 SOC_ALLWINNER_H5		opt_soc.h
 SOC_ALLWINNER_H6		opt_soc.h
+SOC_APPLE_T8103			opt_soc.h
 SOC_BRCM_BCM2837		opt_soc.h
 SOC_BRCM_BCM2838		opt_soc.h
 SOC_BRCM_NS2			opt_soc.h