ARM: dts: aspeed: Add "spi-max-frequency" property

Keep the FMC controller chips at a safe 50 MHz rate and use 100 MHz
for the PNOR on the machines using a AST2500 SoC.

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