MIPS: BCM63XX: add support for "ipsec" clock

This module is only available on BCM6368 so far and does not require
resetting the block.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: mpm@selenic.com
Cc: herbert@gondor.apana.org.au
Patchwork: https://patchwork.linux-mips.org/patch/3324/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>