video: fbdev: atmel_lcdfb: add COMPILE_TEST support

Add COMPILE_TEST support to atmel_lcdfb driver for better compile
testing coverage.

While at it fix improper use of UL (to silence build warnings on
x86_64).

Cc: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>