[ARM] ARMv6: add CPU_HAS_ASID configuration

Presently, we check for the minimum ARM architecture that we're
building for to determine whether we need ASID support.  This is
wrong - if we're going to support a range of CPUs which include
ARMv6 or higher, we need the ASID.

Convert the checks to use a new configuration symbol, and arrange
for ARMv6 and higher CPU entries to select it.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/include/asm-arm/mmu_context.h b/include/asm-arm/mmu_context.h
index 4981ad4..6913d02 100644
--- a/include/asm-arm/mmu_context.h
+++ b/include/asm-arm/mmu_context.h
@@ -20,7 +20,7 @@
 
 void __check_kvm_seq(struct mm_struct *mm);
 
-#if __LINUX_ARM_ARCH__ >= 6
+#ifdef CONFIG_CPU_HAS_ASID
 
 /*
  * On ARMv6, we have the following structure in the Context ID: