pwm: Strengthen dependency for PWM_SIFIVE

Back when the sifive pwm driver was added there was no symbol for sifive
SoCs yet. Today there is ARCH_SIFIVE however. Let PWM_SIFIVE depend on
that to ensure the driver is only build for platforms where there is a
chance that the hardware is available.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20250127105001.587610-2-u.kleine-koenig@baylibre.com


Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>