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 ce62e57fc571 ("ARM: boot0 hook: remove macro,
include whole header file").

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 225f57e..7b618d6 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -287,9 +287,8 @@
 	bool "prepare BOOT0 header"
 	help
 	  If the SoC's BOOT0 requires a header area filled with (magic)
-	  values, then choose this option, and create a define called
-	  ARM_SOC_BOOT0_HOOK which contains the required assembler
-	  preprocessor code.
+	  values, then choose this option, and create a file included as
+	  <asm/arch/boot0.h> which contains the required assembler code.
 
 config ARM_CORTEX_CPU_IS_UP
 	bool