tty: serial: meson: Added S4 SOC compatibility

Make UART driver compatible with S4 SOC UART. Meanwhile, the S4 SOC
UART uses 12MHz as the clock source for baud rate calculations.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Yu Tu <yu.tu@amlogic.com>
Link: https://lore.kernel.org/r/20220422111320.19234-3-yu.tu@amlogic.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>