serial: CONFIG_UART_USE_RUNTIME_CONFIGURE on API for cfg funcs

This adds the CONFIG_UART_USE_RUNTIME_CONFIGURE guard to
disable the API function pointers if the kconfig is not enabled.
Both .configure and .config_get should only be usable if runtime
(re-)configuration of UART is needed.

Ifdef guards are added to drivers previously lacking this guard.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>