microblaze: dts: replace 'linux,stdout-path' with 'stdout-path'

'linux,stdout-path' has been deprecated for some time in favor of
'stdout-path'. Now dtc will warn on occurrences of 'linux,stdout-path'.
Search and replace the one occurrence with 'stdout-path'.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michal Simek <monstr@monstr.eu>
Cc: devicetree@vger.kernel.org
Signed-off-by: Michal Simek <michal.simek@xilinx.com>