MAINTAINERS: Update file list for ARM Freescale IMX

MAINTAINERS: Update file list for ARM Freescale IMX

The MAINTAINERS file currently lists files in
arch/arm/include/asm/arch-imx/ being part of the IMX maintainers
purview, however the arch/arm/include/asm/ directory also contains the
directories arch-imx8, arch-imx8m, arch-imx8ulp and arch-imxrt which
would also appear to be relevant to the team. Tweak the entry to cover
these directories so that tools like get_maintainers.pl will suggest
relevant maintainers when making changes just in these directories.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>