spi: img-spfi: Implement a prepare_message() callback

In preparation for switching to using the SPI core's CS GPIO handling,
move setup of the PORT_STATE register, which must be configured before
CS is asserted, to a prepare_message() callback.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>