arm64: booti: add missing unmap_sysmem()

arm64: booti: add missing unmap_sysmem()

Make sure to call unmap_sysmem() for address allocated by map_sysmem()
before leaving the function; however this patch gives no impact on
the behavior because map_sysmem()/unmap_sysmem() does nothing except
on Sandbox.  Sandbox never runs this code because "booti" is a command
for booting ARM64 kernel image.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>