generic: add b53 swconfig switch driver
Add swconfig switch driver for Broadcom BCM53XX switch chips. Supports
switches connected through MDIO, SPI or memory mapped registers, and
supports BCM5325, BCM539x, BCM531x5 and the BCM63XX internal switch
chips.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: José Vázquez <pepvf@mixmail.com>