backlight: ili9320: add missing SPI dependency

Add this missing SPI dependency and prevent the driver from building
without SPI, because functions of the spi driver are used in this
driver.

  drivers/video/backlight/ili9320.c:51: undefined reference to `spi_sync'

Also, a prompt string for CONFIG_LCD_ILI9320 is added for explicit
selection.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>