xen/arm: Kconfig: Fix XEN_START_ADDRESS input prompt

Remove the part about platform defined address which is not true. The
help text is correct i.e. 0xFFFFFFFF is used as default value to indicate
that user has not customized this address.

Amends: d736b6eb45 ("xen/arm: mpu: Define Xen start address for MPU systems")
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Acked-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/Kconfig

@@ -28,7 +28,7 @@ config ARCH_DEFCONFIG
 	default "arch/arm/configs/arm64_defconfig" if ARM_64
 
 config XEN_START_ADDRESS
-	hex "Xen start address: keep default to use platform defined address"
+	hex "Xen start address"
 	default 0xFFFFFFFF
 	depends on MPU
 	help