arm: socfpga: vining: Unmount UBIFS and detach UBI in ubi_load script

Clean up in ubiload script. Unmount UBIFS from which kernel image was
loaded and detach UBI on which the UBIFS is located, otherwise message
similar to the following is printed just before booting kernel:

Removing MTD device #7 (rootfs) with use count 1
Error when deleting partition "rootfs" (-16)

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Siew Chin Lim <elly.siew.chin.lim@intel.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tien Fong Chee <tien.fong.chee@intel.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@intel.com>