ARM: BCM63xx: Remove custom secondary_startup function

With commit 02b4e275 ("ARM: v7 setup
function should invalidate L1 cache"), the default secondary_startup
function for ARMv7 CPUs does invalidate the L1 cache, which was the sole
reason why BCM63xx had to have its own secondary_startup implementation.

Now that the secondary_startup takes care of this, we can completely
remove that code.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>