board: sl28: add basic PSCI implementation

For now, this only provides reset and poweroff functions.

Signed-off-by: Michael Walle <michael@walle.cc>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/board/kontron/sl28/Makefile b/board/kontron/sl28/Makefile
index 5d220f0..084c11d 100644
--- a/board/kontron/sl28/Makefile
+++ b/board/kontron/sl28/Makefile
@@ -6,6 +6,8 @@
 
 obj-y += common.o ddr.o
 
+obj-$(CONFIG_ARMV8_PSCI) += psci.o
+
 ifdef CONFIG_SPL_BUILD
 obj-y += spl.o
 obj-$(CONFIG_SPL_ATF) += spl_atf.o