MAINTAINERS: Add i.MX8MP Ultra-MACH SBC to ULTRATRONIK BOARD SUPPORT

MAINTAINERS: Add i.MX8MP Ultra-MACH SBC to ULTRATRONIK BOARD SUPPORT

The i.MX8MP-based Ultra-MACH SBC is supported by Ultratronik, and its
device tree has been added under arch/arm64/boot/dts/freescale/.

To ensure proper maintainer coverage and notification of relevant
changes, add the imx8mp-ultra-mach-sbc.dts file to the existing
ULTRATRONIK BOARD SUPPORT section.

This follows the established pattern already used for the STM32MP157C-
based Ultra-FLY SBC.

Signed-off-by: Goran Rađenović <goran.radni@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>