ARM: imx: mach-imx6ul: add imx6ulz support

imx6ulz is derived SoC from imx6ull.
Currently, the boards based on imx6ulz are correctly recognized only if
the compatible includes "fsl,imx6ull", while "fsl,imx6ulz" is ignored.

Signed-off-by: Pierluigi Passaro <pierluigi.p@variscite.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>