MAINTAINERS: add entry for Denali NAND controller driver

The Denali NAND controller driver (drivers/mtd/nand/denali*) has been
largely reworked by me.  Add myself as its maintainer now.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>