board: sifive: unmatched: enable booting on a second NVME device

The HiFive Unmatched board has a M2 slot for NVME and a PCIe slot that
can also be used for NVME. Enable support for a second NVME device, so
that software RAID-1 configurations can be supported at the u-boot
level.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>