ARM: drop "optional" from target select in favor of ARCH_VERSATILE

Since commit a26cd049 ("arch: Make board selection choices
optional"), Kconfig could create such an insane .config file that
no SoC/board is selected.

This is now a real problem for Buildroot, for example.
(http://lists.busybox.net/pipermail/buildroot/2015-July/135125.html

)

This commit drops the "optional" from the ARM target select menu
in favor of "Versatile family".

Rationale:
 - Historically, Linux chose versatile_defconfig as the default
   of ARM defconfig. (arch/arm/Makefile of Linux describes:
   KBUILD_DEFCONFIG := versatile_defconfig)

 - It was published by ARM Ltd.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>