i2c: pcf8575: Add support for 8 bit I2C IO expanders (like pca8574)

This patch add support for using NXP's pca8574 I2C IO expander, which
has only 8 IO lines.

After this change the .data member's information from struct udevice_id
are used to either sent one or two bytes.

Moreover, the '_le16' suffix from pcf8575_i2c_{write|read}_le16()
functions have been removed as now we also sent 8 bit data.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>