drivers: uart: cc13xx_cc26xx: use PRE_KERNEL_1 init level for all UARTs

There does not seem to be a simple way to express the fact that UART1
can be initialized later at the POST_KERNEL level. The existing code
is wrong in the sense that it always initializes DT instance 1 later,
instead of doing it for UART1 (which may not be instance 1).

In addition, UART instances on other platforms are
also initialized at the PRE_KERNEL_1 level regardless of the instance
index, so let's do the same on this platform for consistency.

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