imx8: Add a default reset_cpu() implementation

Add a weak default reset_cpu() implementation just like
it is done on arch/arm/mach-imx/cpu.c.

This allows the removal of the empty reset_cpu() in several
board files.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Tested-by: Hiago De Franco <hiago.franco@toradex.com> # Toradex Colibri iMX8X
diff --git a/arch/arm/mach-imx/imx8/cpu.c b/arch/arm/mach-imx/imx8/cpu.c
index 0b91e44..6e64318 100644
--- a/arch/arm/mach-imx/imx8/cpu.c
+++ b/arch/arm/mach-imx/imx8/cpu.c
@@ -84,6 +84,10 @@
 	}
 }
 
+__weak void reset_cpu(void)
+{
+}
+
 int arch_cpu_init(void)
 {
 #if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_RECOVER_DATA_SECTION)