nfp: nfpcore: add rtsym writing function

Add an rtsym API function that combines the lookup of a symbol and the
writing of a value to it. Values can be written as unsigned 32 or 64 bits.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>