EDAC/sifive: Add EDAC platform driver for SiFive SoCs

Add an EDAC driver for SiFive SoCs. The initial version supports ECC
event monitoring and reporting through the EDAC framework for the SiFive
L2 cache controller. It registers for notifier events from the L2 cache
controller driver (arch/riscv/mm/sifive_l2_cache.c) for L2 ECC events.

 [ bp: Massage commit message. ]

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: James Morse <james.morse@arm.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: linux-riscv@lists.infradead.org
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: sachin.ghadi@sifive.com
Link: https://lkml.kernel.org/r/1557142026-15949-2-git-send-email-yash.shah@sifive.com