mfd: dbx500-prcmu: Drop DSI pll clock functions

The DSI PLLs are handled by the generic clock framework
since ages, this code is completely unused and misleading.
Delete it.

Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>