mmc: sdhci: Enhance debug register dump

Add new host operation dump_vendor_regs to provide a
mechanism through which host drivers can dump vendor
specific registers in addition to SDHC registers
during error conditions.

Change-Id: Ifba3ddcb18c3c0917343d99fe58d5ed04b2da871
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>