bcm53xx: include Broadcom PHY driver in the kernel

Initial idea was to use package with this PHY driver for devices that
need it. Unfortunately this can't work as bgmac is built-in and PHY
probing happens before loading modules - it results in PHY subsystem
picking default (generic) PHY driver.

There were two ways of solving this:
1) Making bcm53xx use bgmac as module
xor
2) Built-in Broadcom PHY driver

After some quick discussion it seems we can simply built-in the driver
as increased kenel size is relatively small (1805 B).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>