backlight: corgi_lcd: fix missing prototype

The corgi_lcd_limit_intensity() function is called from platform
and defined in a driver, but the driver does not see the declaration:

drivers/video/backlight/corgi_lcd.c:434:6: error: no previous prototype for 'corgi_lcd_limit_intensity' [-Werror=missing-prototypes]
  434 | void corgi_lcd_limit_intensity(int limit)

Move the prototype into a header that can be included from both
sides to shut up the warning.

Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>