lcd: atmel: move atmel-specific fb_put_word to atmel_lcdfb

Reduce the amount of platform-specific code in common/lcd.c by moving Atmel
implementation of fb_put_word() to atmel_lcdfb.c. Since we must also have a
default implementation for everybody else, make the remainder of the code
into a weak function.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Josh Wu <josh.wu@atmel.com>
Cc: Bo Shen <voice.shen@atmel.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Anatolij Gustschin <agust@denx.de>