bus: imx-weim: improve error handling upon child probe-failure

Probe all children of the WEIM node, reporting any failures.  Report
failure from parsing of WEIM node itself if probes of all children fail.

Signed-off-by: Alison Chaiken <alison_chaiken@mentor.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>