uart: sifive: align txcnt of UART1 with UART0

The UART1 is not actually accessible on the HiFive1 since the
QFN48 package for the FE310, used by this board, does not expose the
required pads. However, it is still possible to use the UART1 on HiFive1
emulators (e.g. using UART0 for debugging and UART1 for SLIP).

Unfortunately, the UART0 and UART1 currently have different default
configurations when it comes to the configured transmission watermark
(txcnt). With a txcnt of zero (the default for UART1) the UART is not
actually usable (see #18118). Contrary to UART0 (see #23699), the UART1
does therefore not work by default on Zephyr which took me some time to
figure out.

This commit aligns the default UART1 configuration with the UART0,
thereby making it work by default.

Signed-off-by: Sören Tempel <tempel@uni-bremen.de>