mfd: db8500-prcmu: Handle missing FW variant

There was an "unknown" firmware variant turning up in the wild
causing problems in the clock driver. Add this missing variant
and clarify that varian 11 and 15 are Samsung variants, as this
is now very well known from released products.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>