ARM: BCM: Enable ARM erratum 798181 for BRCMSTB

Commit 04fcab32 ("ARM: 8111/1: Enable
erratum 798181 for Broadcom Brahma-B15") enables this erratum for
affected Broadcom Brahma-B15 CPUs when CONFIG_ARM_ERRATA_798181=y.
Let's make sure that config option is actually set.

Signed-off-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Olof Johansson <olof@lixom.net>