cbus: Fix few compile errors found with randconfig

drivers/cbus/retu-pwrbutton.c:98: undefined reference to `input_allocate_device'
drivers/cbus/retu-pwrbutton.c:109: undefined reference to `input_register_device'
drivers/cbus/retu-pwrbutton.c:118: undefined reference to `input_free_device'
drivers/built-in.o: In function `retubutton_remove':
drivers/cbus/retu-pwrbutton.c:135: undefined reference to `input_unregister_device'
drivers/cbus/retu-pwrbutton.c:136: undefined reference to `input_free_device'
...

drivers/cbus/retu-wdt.c:167:3: error:
implicit declaration of function 'cpu_is_omap16xx' [-Werror=implicit-function-declaration]
drivers/cbus/retu-wdt.c:170:3: error:
implicit declaration of function 'cpu_is_omap24xx' [-Werror=implicit-function-declaration]

Signed-off-by: Tony Lindgren <tony@atomide.com>