libfdt: fix SPDX license identifiers

License identifiers should be machine readable.

According to the SPDX v2.3.0 specification annex E parentheses are not
used in the SPDX identifier field when specifying multiple licenses [1].

[1] https://spdx.github.io/spdx-spec/v2.3/using-SPDX-short-identifiers-in-source-files/



Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Anup Patel <anup@brainfault.org>