drm/pl111: pl111_vexpress.c: Add of_node_put() before return

Each iteration of for_each_available_child_of_node puts the previous
node, but in the case of a break from the middle of the loop there is
no put, thus causing a memory leak. Hence add an of_node_put before the
break.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190706132742.3250-1-nishkadg.linux@gmail.com