pwm: pwm-mediatek: Pass PWM_CK_26M_SEL from platform data

pwm: pwm-mediatek: Pass PWM_CK_26M_SEL from platform data

In preparation for adding support for new SoCs, remove variable
has_ck_26m_sel from pwm_mediatek_of_data and replace it with a
u16 pwm_ck_26m_sel_reg, meant to hold the register offset for
PWM_CK_26M_SEL.

Also, since the reg offset is guaranteed to never be zero, the
logic to check for "has_ck_26m_sel" is changed to check if the
register offset in pwm_ck_26m_sel_reg is more than zero.

Analogously, when writing, use the register offset from platform
data instead of using the PWM_CK_26M_SEL definition.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20250623120118.109170-3-angelogioacchino.delregno@collabora.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>