mmc: renesas_sdhi: document version of RZ/A1 instance

We don't use this new define yet, but it is helpful to document which
versions we know of.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Chris Brandt <Chris.Brandt@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>