nfp: move mutex code out of nfp_cppcore.c

After mutex cache removal we can put the mutex code in a separate
source file.  This makes it clear it doesn't play with internals
of struct nfp_cpp any more.

No functional changes.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>