xtensa: make MMU-related configuration options depend on MMU

MMUv3 and HIGHMEM support are available only on configurations with MMU,
don't show them otherwise.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Kconfig

@@ -197,6 +197,7 @@ config MATH_EMULATION
 
 config INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
 	bool "Initialize Xtensa MMU inside the Linux kernel code"
+	depends on MMU
 	default y
 	help
 	  Earlier version initialized the MMU in the exception vector
@@ -226,6 +227,7 @@ config INITIALIZE_XTENSA_MMU_INSIDE_VMLINUX
 
 config HIGHMEM
 	bool "High Memory Support"
+	depends on MMU
 	help
 	  Linux can use the full amount of RAM in the system by
 	  default. However, the default MMUv2 setup only maps the