riscv: dts: hifive-unleashed-a00: Add -u-boot.dtsi

Add U-Boot specific dts file for hifive-unleashed-a00, this
would help to add u-boot specific properties and other node
changes without touching the base dts(i) files which are easy
to sync from Linux.

Added spi2 alias for qspi2 as an initial u-boot specific
property change.

spi probing in current dm model is very much rely on aliases
numbering. Even though the qspi2 can't come under any associated
spi nor flash it would require to specify the same to make proper
binding happen for other spi slaves.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Sagar Kadam <sagar.kadam@sifive.com>