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: Marek BehĂșn <marek.behun@nic.cz>
diff --git a/arch/arm/mach-keystone/Makefile b/arch/arm/mach-keystone/Makefile
index 3e076e1..6c7c250 100644
--- a/arch/arm/mach-keystone/Makefile
+++ b/arch/arm/mach-keystone/Makefile
@@ -9,6 +9,7 @@
 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