[ARM] pxa/hx4700: actually use platform_lcd driver

Commit e2c509c7 ([ARM] pxa/hx4700: use platform_lcd driver)
missed to actually register platform device for LCD.
It causes following GCC warning:
arch/arm/mach-pxa/hx4700.c:553: warning: 'hx4700_lcd' defined but not used

Signed-off-by: Dmitry Artamonow <mad_soft@inbox.ru>
Acked-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>