mmc: core: Add a new debugfs entry.

Added a new debugfs entry to support for crashing
the system upon an error.
The default value of this debugfs entry is 'N'.
This  need to set true('Y'), to crash upon an error.

Path to this debugfs entry to echo 'Y'/'N' is
/sys/kernel/debug/mmc0/crash_on_err for eMMC and
/sys/kernle/debug/mmc1/crash_on_err for SD card.

Change-Id: Ie7c79fb235b4882d73bb10193930418bd614ceaa
Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>