MAINTAINERS: ARM: at91: update entry for ARM/Microchip

Change the entry name and move it to its alphabetical location.
We move to ARM/Microchip instead of ARM/ATMEL to reflect the merger
that is now nearly 2 years old.
AT91 is kept as the identity of our community. The atmel pattern is
obviously kept as well. I removed the names of the different SoCs as
they are better exposed in the arch/arm/mach-at91/Kconfig entries.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>