ARM: dts: BCM5301X: Specify flash partitions

Most devices use Broadcom standard partitions which allows them to be
described with the "brcm,bcm947xx-cfe-partitions". Exceptions are:
1) TP-LINK devices which use "os-image" partition with TRX containing
   kernel only + separated rootfs partition.
2) Asus RT-AC87U with custom "asus" partition.

This commit also removes undocumented and unsupported linux,part-probe
binding which got accidentally upstreamed while describing SPI
controller.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>