serial: Change Kconfig entry for CLPS711X-target

This trivial patch adds a short description for SERIAL_CLPS711X Kconfig
entry, removes excess dependence on the ARM-platform (this is done
globally for the platform), allows the driver to be compiled by default
and removes unnecessary description about GRUB and LILO, because these
bootloaders do not supported this platform.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>