serial: uart_liteuart: Use LiteX HAL
serial: uart_liteuart: Use LiteX HAL

Use LiteX HAL functions instead of `sys_read*` or `sys_write*`
functions.
They use them inside, but choose which one to use according to
configured CSR data width.

Signed-off-by: Michal Sieron <efb49a7c59124393d61f00761aa9acfb90220a86@internships.antmicro.com>

galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr