Kconfig: Remove trailing whitespaces in prompt

Remove additional trailing whitespaces in prompt reported by kconfiglib:

warning: DM_PMIC_SANDBOX (defined at drivers/power/pmic/Kconfig:133) has
leading or trailing whitespace in its prompt
warning: <choice> (defined at dts/Kconfig:204) has leading or trailing
whitespace in its prompt

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Felix Brack <fb@ltec.ch>