dt-bindings: iio: adc: ti,tlc4541 binding conversion

Simple binding so easy to convert.
Dropped the stated value of maximum spi bus frequency as it does
not seem to correspond to the datasheet.  The value of 200kHz
is the max sampling frequency of the ADC, not the clock frequency of
the SPI bus.

Added #io-channel-cells to allow use as a provider of channels to
other devices via the consumer binding.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-By: Phil Reid <preid@electromag.com.au>
Cc: Phil Reid <preid@electromag.com.au>
Link: https://lore.kernel.org/r/20200809111753.156236-14-jic23@kernel.org