ARM: dts: BCM63xx: fix L2 cache properties

The L2 cache properties were completely off with respect to what the
hardware is configured for. Fix the cache-size, cache-line-size and
cache-sets to reflect the L2 cache controller we have: 512KB, 16 ways
and 32 bytes per cache-line.

Fixes: 46d4bca0

 ("ARM: BCM63XX: add BCM63138 minimal Device Tree")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>