platform: Add Spike initial support

This patch adds initial platform support Spike emulator.

Signed-off-by: James Clarke <jrtc27@jrtc27.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>

lib/utils/sys/objects.mk

@@ -8,3 +8,4 @@
 #
 
 libsbiutils-objs-y += sys/clint.o
+libsbiutils-objs-y += sys/htif.o