MIPS: BCM47XX: Add support for Netgear R6200 V1

The Netgear R6200 v1 uses a BCM4718A1 SOC and a BCM4352/BCM4360 for 5GHz
wireless. This patch adds support for detecting this model board and
registers the 3 buttons.

I have tested that the device can boot kernels 4.14 and 4.19 under
OpenWRT.

There is one issue that the LEDs on the device are controlled by a
74HC164 that uses bit-banging instead of SPI so it isn't accessible to
the kernel without adding a workaround. Without any workaround the
device on boot will flash all LEDs once then the power LED will remain
amber as all other LEDs stay off.

Signed-off-by: Edward Matijevic <motolav@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
[paul.burton@mips.com:
  - Sort bcm47xx_board_list_board_id alphabetically by board type.
  - Fix whitespace.
  - Wrap commit message & drop OpenWRT-based justification for
    bcm47xx_board_list_board_id being mis-sorted.]
Signed-off-by: Paul Burton <paul.burton@mips.com>