ARM: fix LTO for keystone

When building keystone with LTO the compiler complains:
  Error: selected processor does not support `smc #0' in Thumb mode

Fix this by removing -flto for the file implementing these SMC calls.
Signed-off-by: NMarek Behún <marek.behun@nic.cz>

arch/arm/mach-keystone/Makefile

@@ -9,6 +9,7 @@ obj-y	+= init.o
 obj-y	+= psc.o
 obj-y	+= clock.o
 obj-y	+= mon.o
+CFLAGS_REMOVE_mon.o := $(LTO_CFLAGS)
 ifndef CONFIG_SPL_BUILD
 obj-y	+= cmd_clock.o
 obj-y	+= cmd_mon.o