iio: addac: stx104: Add 8254 Counter/Timer support

iio: addac: stx104: Add 8254 Counter/Timer support

The STX104 features an 8254 Counter/Timer chip providing three
counter/timers which can be used for frequency measurement, frequency
output, pulse width modulation, pulse width measurement, event count,
etc. The STX104 provides a register bank selection to bank select
between the 8254 Bank and the Indexed Register Array Bank; the Indexed
Register Array is not utilized by this driver, so the 8254 Bank is
selected unconditionally.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Link: https://lore.kernel.org/r/20230916112031.3634-1-william.gray@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>