drivers: uart: cc13xx_cc26xx: set power config based on UART base addr

The power configuration is dependent on which UART is physically used.
In order to allow DT_INST_FOREACH_STATUS_OKAY() to iterate through
instances without the assumption that index 0 corresponds to UART0
(which would be incorrect in the case when only UART1 is enabled),
we need to check the base address to identify which UART is being dealt
with.

Signed-off-by: Vincent Wan <vincent.wan@linaro.org>