config, dev-arm: Fix UART handling baremetal mode

config, dev-arm: Fix UART handling baremetal mode

fs.py in baremetal mode currently fails for the VExpress_GEM5_V1 platform
due to inconsistent UART naming with error message:

AttributeError: object 'VExpress_GEM5_V1' has no attribute 'uart'

Consistently name keep all UARTs in the Arm platforms in a vector named
'uart' or as a single device named 'uart'. Update the configuration
scripts to reflect the fact that 'uart' can be a vector.

Change-Id: I20b8dbac794d6a9be19b6ce8c335a097872132fb
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/12473
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>