[ARM] pxa/hx4700: use platform_lcd driver

Instead of directly registering an lcd_device, use the platform_lcd driver.
This fixes a broken build in case CONFIG_LCD_CLASS_DEVICE is not enabled.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Cc: Ben Dooks <ben-linux-arm@fluff.org>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>