ARM: dts: aspeed: add SPI controller bindings

Let's define the SPI controllers in the Aspeed SoCs AST2500 and
AST2400 and also enable these, as well as the chips, on the associated
platforms.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>