microblaze: Drop architecture-specific declaration of early_printk

miceroblaze:allmodconfig fails to build, complaining that
early_printk is redeclared.

include/linux/printk.h:114:6: error: static declaration of 'early_printk'
		follows non-static declaration
 void early_printk(const char *s, ...) { }
      ^
In file included from arch/microblaze/include/asm/page.h:19:0,
                 from arch/microblaze/include/asm/io.h:15,
                 from include/linux/io.h:22,
                 from kernel/irq/generic-chip.c:6:
arch/microblaze/include/asm/setup.h:22:6:
		note: previous declaration of 'early_printk' was here
 void early_printk(const char *fmt, ...);

This happens because CONFIG_EARLY_PRINTK is not enabled in this
configuration.

The architecture-specific declaration is not needed; drop it.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>