tpm: Reorganize the I2C TPM driver

This patch does a similar code reogranzation from
  http://patchwork.ozlabs.org/patch/132179/


which is based on an old version of code (fdt support and bus selection
still not in). It merges this tidy-up on top of the recent code. It does
not make any logical change.

tpm.c implements the interface defined in tpm.h based on underlying
LPC or I2C TPM driver. tpm.c and the underlying driver communicate
throught tpm_private.h.

Note: Merging the LPC driver with tpm.c is left to future patches.

Change-Id: Ie1384f5f9e3935d3bc9a44adf8de80c5a70a5f2b
Signed-off-by: Tom Wai-Hong Tam <waihong@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>