pwm: meson: Change MISC_CLK_SEL_WIDTH to MISC_CLK_SEL_MASK

MISC_CLK_SEL_WIDTH is only used in one place where it's converted into
a bit-mask. Rename and change the macro to be a bit-mask so that
conversion is not needed anymore. No functional changes intended.

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>