arm: update Kconfig help for ENABLE_ARM_SOC_BOOT0_HOOK

When this config is enabled, <asm/arch/boot0.h> is included,
but the Kconfig help says this should be a define
(ARM_SOC_BOOT0_HOOK).

The code for this in arch/arm/lib/vectors.s was changed on
01/02/2017 with commit ce62e57f ("ARM: boot0 hook: remove macro,
include whole header file").

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>