ARM: dts: aspeed: Rename flash-controller nodes

The device tree compiler has started spitting out warnings about these
names, insisting they be called 'spi':

 ../arch/arm/boot/dts/aspeed-g5.dtsi:108.35-128.5: Warning
 (spi_bus_bridge): /ahb/flash-controller@1e631000: node name for SPI
 buses should be 'spi'

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