mfd: db8500-prcmu: Check return of devm_ioremap for error

Error check around return value of devm_ioremap is missing. Add the same
to avoid NULL pointer dereference.

Signed-off-by: Pramod Gurav <pramod.gurav@smartplayin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>