cmake: dts: Fix passing multiple EXTRA_DTC_FLAGS to gen_defines.py

Since EXTRA_DTC_FLAGS will be ';' seperated if we pass that as is if
there are multiple items in the list we'll get an error.  Sanitize
EXTRA_DTC_FLAGS by replacing ';' with ' ' before we call gen_defines.py

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>