Kconfig: Remove trailing whitespace in its prompt

All errors are generated by ./tools/qconfig.py -b -j8 -i whatever.
Error look like this:
warning: SPL_CLK_CCF (defined at drivers/clk/Kconfig:59) has leading or
trailing whitespace in its prompt

Signed-off-by: Michal Simek <michal.simek@amd.com>