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

As follow-up to "dt: introduce of_get_child_by_name to get child node by
name." patch, This patch removes some of the code duplication in the
driver by replacing it with of_get_child_by_name instead.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>