riscv: dts: canaan: Fix SPI FLASH node names

"make dtbs_check":

    arch/riscv/boot/dts/canaan/sipeed_maix_bit.dt.yaml: spi-flash@0: $nodename:0: 'spi-flash@0' does not match '^flash(@.*)?$'
	    From schema: Documentation/devicetree/bindings/mtd/jedec,spi-nor.yaml

Fix this by renaming all SPI FLASH nodes to "flash".

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Tested-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>