kboot: MI part of the memory enumeration code

enumerate_memory_arch is called once early in kboot's startup to allow
us to discover the memory layout, reserved areas, etc of the system
memory. Add the MI interface part of this.

Sponsored by:		Netflix
Reviewed by:		tsoome, kevans
Differential Revision:	https://reviews.freebsd.org/D38247

stand/kboot/kboot.h

@@ -9,6 +9,15 @@
 
 #define DEVT_HOSTDISK 1234
 
+struct memory_segments
+{
+	uint64_t	start;
+	uint64_t	end;
+	uint64_t	type;	/* MD defined */
+};
+
+bool enumerate_memory_arch(void);
+
 bool has_acpi(void);
 vm_offset_t acpi_rsdp(void);