ARM: keystone2: Switch to using the poweroff command

Now that we have a standard way to power off the hardware, switch to
using that rather than our own command.

Cc: Vitaly Andrianov <vitalya@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 653ecc8..e5f57ef 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -450,6 +450,7 @@
 	bool "TI Keystone"
 	select CPU_V7
 	select SUPPORT_SPL
+	select CMD_POWEROFF
 
 config ARCH_MX7
 	bool "Freescale MX7"