microblaze: remove undefined of_get_cpu_node declaration

This patch removes the declaration of the function 'of_get_cpu_node'
which is not defined for microblaze. This is in preparation to move
it's definition from PPC to DT common code.

Michal Simek says: "it was just there because Microblaze
was based on powerpc code"

Acked-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>