serial: sifive: enable clocks for UART when probed

When the PWM driver was changed to disable clocks if no PWMs are enabled,
it ended up also disabling the shared parent with the UART, since the
UART doesn't do any clock enablement on its own.

To avoid these surprises, switch to clk_get_enabled().

Fixes: ace41d75

 ("pwm: sifive: Ensure the clk is enabled exactly once per running PWM")
Cc: stable <stable@kernel.org>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Olof Johansson <olof@lixom.net>
Link: https://lore.kernel.org/r/20220920160017.7315-1-olof@lixom.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>