common/sfc_efx/base: update registers check for Riverhead

common/sfc_efx/base: update registers check for Riverhead

Riverhead family got letter 'G' for registers definition.
Registers used for Riverhead should have the letter in the
specification range.

Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>
Reviewed-by: Vijay Kumar Srivastava <vsrivast@xilinx.com>