ARM: AM437x: Align HS device variant defconfig filename

Align the name of the defconfig file for high-security (HS) device variants
from the AM43xx family of SoCs with the corresponding name used for the
general purpose devices. This allows for easier cross-association of those
files and also provides room to grow from an HS device part number
perspective.

Furthermore, update and cleanup associated MAINTAINERS file.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: Madan Srinivas <madans@ti.com>