ARM: remove redudant information from Kconfig files

- "string" type for SYS_* is defined in arch/Kconfig
 - SYS_CPU "armv7" has been replaced with "select CPU_V7"
 - SYS_SOC "tegra124" is already defined in tegra124/Kconfig

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Bo Shen <voice.shen@atmel.com>