Makefile: Minor fix in OPENSBI_VERSION_GIT

Currently, if someone has forked OpenSBI repo quite sometime back
and this fork is not having updated tags from upstream riscv/opensbi
repo then "git describe" command can fail. To tackle this, we redirect
error output of "git describe" to /dev/null.

Signed-off-by: Anup Patel <anup.pate@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>