MAINTAINERS: Rename 'Microchip Platforms' to 'Microchip MEC Platforms'

This commit renames the 'Microchip Platforms' entry to 'Microchip
MEC Platforms' since this entry is specific to the Microchip MEC family
devices.

This is in preparation for adding an entry for the Microchip (formerly
Atmel) SAM family devices.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>