README: Update pylibfdt install instructions

Now that pip is supported for installs, update the install instructions to
use it. Using pip over setup.py is generally recommended and simpler.

Also, drop 'SETUP_PREFIX' as it doesn't exist anywhere.

Signed-off-by: Rob Herring <robh@kernel.org>
Message-Id: <20211112041633.741598-2-robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
1 file changed