xtensa: clean up Kconfig dependencies for custom cores

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Kconfig

@@ -63,9 +63,7 @@ config TRACE_IRQFLAGS_SUPPORT
 	def_bool y
 
 config MMU
-	bool
-	default n if !XTENSA_VARIANT_CUSTOM
-	default XTENSA_VARIANT_MMU if XTENSA_VARIANT_CUSTOM
+	def_bool n
 
 config VARIANT_IRQ_SWITCH
 	def_bool n
@@ -73,9 +71,6 @@ config VARIANT_IRQ_SWITCH
 config HAVE_XTENSA_GPIO32
 	def_bool n
 
-config MAY_HAVE_SMP
-	def_bool n
-
 menu "Processor type and features"
 
 choice
@@ -102,7 +97,6 @@ config XTENSA_VARIANT_DC233C
 
 config XTENSA_VARIANT_CUSTOM
 	bool "Custom Xtensa processor configuration"
-	select MAY_HAVE_SMP
 	select HAVE_XTENSA_GPIO32
 	help
 	  Select this variant to use a custom Xtensa processor configuration.
@@ -128,6 +122,7 @@ config XTENSA_VARIANT_MMU
 	bool "Core variant has a Full MMU (TLB, Pages, Protection, etc)"
 	depends on XTENSA_VARIANT_CUSTOM
 	default y
+	select MMU
 	help
 	  Build a Conventional Kernel with full MMU support,
 	  ie: it supports a TLB with auto-loading, page protection.
@@ -145,7 +140,7 @@ source "kernel/Kconfig.preempt"
 
 config HAVE_SMP
 	bool "System Supports SMP (MX)"
-	depends on MAY_HAVE_SMP
+	depends on XTENSA_VARIANT_CUSTOM
 	select XTENSA_MX
 	help
 	  This option is use to indicate that the system-on-a-chip (SOC)