ARM: Check ARCH_MULTI_V7 to differentiate ARMv5/v7 Allwinner SoCs

Allwinner also has some ARMv5 SoCs.

In order to add support for them, check ARM_MULTI_V7 before enabling
ARMv7 SoC's. Add help text for ARCH_SUNXI menuconfig.

Signed-off-by: Mesih Kilinc <mesihkilinc@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>