tty: serial: fsl_lpuart: rename register variables more specifically

There are many fuzzy register variables in the lpuart driver, such as
temp, tmp, val, reg. Let's give these register variables more specific
names.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Link: https://lore.kernel.org/r/20250312023904.1343351-4-sherry.sun@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: SherrySun5