ramips: fix RBM11G partitioning

ramips: fix RBM11G partitioning

This patch improves faf64056ddd46992a75b1e277d94541c7251035c by correcting
the partition scheme for the "RouterBoot" section of the flash.

The partition scheme initially submitted is incorrect and does not reflect
the actual flash structure.

The "RouterBoot" section (name matching OEM) is subdivided in several
static segments, as they are on ar71xx RB devices albeit with different
offsets and sizes.
The naming convention from ar71xx has been preserved, except for the
bootloaders which are named "bootloader1" and "bootloader2" to avoid
confusion with the master "RouterBoot" partition.
The preferred 'fixed-partitions' DTS node syntax is used, with nesting
support as introduced in 2a598bbaa3.
"partition" is used for node names, with associated "label" to match
policy set by 6dd94c2781.

Leave a note in DTS to explain how the original author selected the SPI speed.

Tested-by: Tobias Schramm <tobleminer@gmail.com>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
Reviewed-by: Rafał Miłecki <rafal@milecki.pl>