gpio-cdev: provide HASH values for nu801

gpio-cdev: provide HASH values for nu801

Chen Minqiang reported that he has troubles downloading nu801.
His logs showed the followin TLS Handshake failure.

|Checking out files from the git repository...
|Cloning into 'nu801-d9942c0c'...
|fatal: unable to access 'https://github.com/chunkeey/nu801.git/':
|  gnutls_handshake() failed: The TLS connection was non-properly terminated.
|Makefile:39: recipe for target '[...]/dl/nu801-d9942c0c.tar.xz' failed

This can be fixed by providing a PKG_MIRROR_HASH. The download
scripts will now be able to pull the source from OpenWrt's source
archive, which should be available through HTTP.

Reported-by: Chen Minqiang <ptpt52@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>