arm: Open clock gate on UART setup

arm: Open clock gate on UART setup

Add the infrastructure to open a clock gate on UART configuration. This
is particularly helpful if Linux drivers close the gate when releasing
the device.

For now the assumption is that a clock gate can be described by a single
bit in a specific register.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>