Move console definitions into a new console.h file

The console includes a global variable and several functions that are only
used by a small subset of U-Boot files. Before adding more functions, move
the definitions into their own header file.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/powerpc/cpu/mpc8xx/i2c.c b/arch/powerpc/cpu/mpc8xx/i2c.c
index 6146de3..3dff4ab 100644
--- a/arch/powerpc/cpu/mpc8xx/i2c.c
+++ b/arch/powerpc/cpu/mpc8xx/i2c.c
@@ -12,6 +12,7 @@
  */
 
 #include <common.h>
+#include <console.h>
 
 #ifdef CONFIG_HARD_I2C