of: add stub of_get_child_by_name for non-OF builds

Fixes build error on s3c6400_defconfig, introduced by commit
06455bbc

, "dt/s3c64xx/spi: Use
of_get_child_by_name to get a named child".

drivers/spi/spi-s3c64xx.c: In function 's3c64xx_get_slave_ctrldata':
drivers/spi/spi-s3c64xx.c:838:2: error: implicit declaration of function
    'of_get_child_by_name' [-Werror=implicit-function-declaration]

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>