microblaze: remove redundant early_printk support

With earlycon support now enabled, the arch specific early_printk support
can be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>