dt-bindings: mmc: sdhci-of-aspeed: Document Aspeed SD controller

The ASPEED SD/SDIO/MMC controller exposes two slots implementing the
SDIO Host Specification v2.00, with 1 or 4 bit data buses, or an 8 bit
data bus if only a single slot is enabled.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>