mtd: nand: mpc5121: use 'of_machine_is_compatible' to simplify code

The current code is the same as 'of_machine_is_compatible'.
So use it in order to remove a few lines of code and to be more
consistent with other parts of the kernel.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>