mmc: debugfs: add debugfs entry to force raise host errors

The SDHC spec allows to force raise errors that is useful for
debugging error handler routines. Add debugfs entry force_error
to trigger host errors from userspace. Check SDHCI_SET_INT_ERROR
register for error bitmask info.

Usage: echo 0x1 > /sys/kernel/debug/mmcX/force_error
X - denotes the slot id

Change-Id: I9f67442a79b2645cbdc3020d1a10c0b32840ce32
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>