HV: separate e820 related code as e820.c/h

1. e820 is modulized as separated files.
2. move boot_regs into multiboot.h as it's related with
multiboot info header

Tracked-On: #1842
Signed-off-by: Minggui Cao <minggui.cao@intel.com>
Acked-by: Anthony Xu <anthony.xu@intel.com>

hypervisor/arch/x86/cpu.c

@@ -9,6 +9,7 @@
 #include <schedule.h>
 #include <version.h>
 #include <trampoline.h>
+#include <e820.h>
 
 spinlock_t trampoline_spinlock = {
 	.head = 0U,