MAINTAINERS: update arm64 Microchip entries

Krzysztof noticed that patches for arch/arm64/boot/dts/microchip were
getting lost & the listed tree was inactive.

Nicolas and I are willing to shepherd patches to Arnd, using the
existing at91 tree, so add a new entry covering
arch/arm64/boot/dts/microchip, listing us as maintainers.

Drop the tree from the existing sparx5 entry & narrow the devicetree
pattern to just sparx devices, leaving Lars, Steen and Daniel looking
after support for their SoCs.

CC: Rob Herring <robh+dt@kernel.org>
CC: Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>
CC: Conor Dooley <conor@kernel.org>
CC: Nicolas Ferre <nicolas.ferre@microchip.com>
CC: Claudiu Beznea <claudiu.beznea@microchip.com>
CC: soc@kernel.org
CC: Lars Povlsen <lars.povlsen@microchip.com>
CC: Steen Hegelund <Steen.Hegelund@microchip.com>
CC: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Acked-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230522-wagon-stencil-a164ec39322a@wendy
Signed-off-by: Arnd Bergmann <arnd@arndb.de>