distribution.conf: enable device tree runtime self tests

This will add few extra prints to the boot log.
Example log:
[ 9.860954] ### dt-test ### start of selftest - you will see error messages
[ 9.861889] /testcase-data/phandle-tests/consumer-a: could not get #phandle-cells-missing for /testcase-data/phandle-tests/provider1
[ 9.861898] /testcase-data/phandle-tests/consumer-a: could not get #phandle-cells-missing for /testcase-data/phandle-tests/provider1
[ 9.861912] /testcase-data/phandle-tests/consumer-a: could not find phandle
[ 9.861924] /testcase-data/phandle-tests/consumer-a: could not find phandle
[ 9.861935] /testcase-data/phandle-tests/consumer-a: arguments longer than property
[ 9.861945] /testcase-data/phandle-tests/consumer-a: arguments longer than property
[ 9.863717] irq: no irq domain found for /testcase-data/interrupts/intc0 !
[ 9.868324] ### dt-test ### end of selftest - 96 passed, 0 failed

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Andrey Konovalov <andrey.konovalov@linaro.org>