bcm53xx: fix support for TP-Link Archer C5 v2 and C9 v1

bcm53xx: fix support for TP-Link Archer C5 v2 and C9 v1

As it appears, these devices just needed a resize of their os-image
partition. Increase it to 0x300000, so we have some extra space left
for future kernels.

Note that this only adjusts the partioning scheme, but since I
do not own the device I cannot say whether the bootloader will
actually cope with os-image > 2 MiB.

Cc: John Crispin <john@phrozen.org>
Cc: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>