of/irq: Fix potential buffer overflow

Commit 23616132

, "of/irq: Refactor interrupt-map parsing" introduced
a potential buffer overflow bug because it doesn't do sufficient range
checking on the input data. This patch adds the appropriate checking and
buffer size adjustments. If the bounds are out of range then warn
loudly. MAX_PHANDLE_ARGS should be sufficient. If it is not then the
value can be increased.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>