nios2: convert nios2 cpu to driver model

Convert nios2 cpu to driver model. The cpu parameters are
extracted from device tree and saved to global data structure.
We will use them to replace the custom_fpga.h .

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/Kconfig b/arch/Kconfig
index 207c778..9be1538 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -64,6 +64,9 @@
 	select HAVE_GENERIC_BOARD
 	select SYS_GENERIC_BOARD
 	select SUPPORT_OF_CONTROL
+	select OF_CONTROL
+	select DM
+	select CPU
 
 config OPENRISC
 	bool "OpenRISC architecture"