lcd: mpc8xx: move mpc823-specific fb_put_byte to mpc8xx_lcd.c

Reduce the amount of platform-specific code in common/lcd.c by moving MPC823
implementation of fb_put_byte() to mpc8xx_lcd.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>
Tested-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Josh Wu <josh.wu@atmel.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Anatolij Gustschin <agust@denx.de>