mmc: tmio: add references to bit defines in the header

When defining bits, make sure we always have a reference to the register
they belong to. For now, renaming all bits properly seems too intrusive,
so at least make sure we have proper documentation.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>