mmc: sdhci-of-aspeed: Add support for the ASPEED SD controller

Add a minimal driver for ASPEED's SD controller, which exposes two
SDHCIs.

The ASPEED design implements a common register set for the SDHCIs, and
moves some of the standard configuration elements out to this common
area (e.g. 8-bit mode, and card detect configuration which is not
currently supported).

The SD controller has a dedicated hardware interrupt that is shared
between the slots. The common register set exposes information on which
slot triggered the interrupt; early revisions of the patch introduced an
irqchip for the register, but reality is it doesn't behave as an
irqchip, and the result fits awkwardly into the irqchip APIs. Instead
I've taken the simple approach of using the IRQ as a shared IRQ with
some minor performance impact for the second slot.

Ryan was the original author of the patch - I've taken his work and
massaged it to drop the irqchip support and rework the devicetree
integration. The driver has been smoke tested under qemu against a
minimal SD controller model and lightly tested on an ast2500-evb.

Signed-off-by: Ryan Chen <ryanchen.aspeed@gmail.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>