kconfig: use bool instead of boolean for type definition attributes

Linux stopped the use of keyword 'boolean' in Kconfig.

Refer to commit 6341e62b212a2541efb0160c470e90bd226d5496 ("kconfig:
use bool instead of boolean for type definition attributes")
in Linux Kernel.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/arch/arm/cpu/armv8/Kconfig b/arch/arm/cpu/armv8/Kconfig
index acf2460..7e1fc4c 100644
--- a/arch/arm/cpu/armv8/Kconfig
+++ b/arch/arm/cpu/armv8/Kconfig
@@ -1,7 +1,7 @@
 if ARM64
 
 config ARMV8_MULTIENTRY
-        boolean "Enable multiple CPUs to enter into U-Boot"
+        bool "Enable multiple CPUs to enter into U-Boot"
 
 config ARMV8_SPIN_TABLE
 	bool "Support spin-table enable method"