xtensa: move asid_cache from fault.c to mmu.c

asid_cache is only useful with full MMU, but fault.c is also useful with
MPU. Move asid_cache definition to MMU-specific source file.

Signed-off-by: Max Filippov <[e-mail]>

arch/xtensa/mm/fault.c

···
 #include <asm/cacheflush.h>
 #include <asm/hardirq.h>
 
-DEFINE_PER_CPU(unsigned long, asid_cache) = ASID_USER_FIRST;
 void bad_page_fault(struct pt_regs*, unsigned long, int);
 
 /*

arch/xtensa/mm/mmu.c

···
 #include <asm/initialize_mmu.h>
 #include <asm/io.h>
 
+DEFINE_PER_CPU(unsigned long, asid_cache) = ASID_USER_FIRST;
+
 #if defined(CONFIG_HIGHMEM)
 static void * __init init_pmd(unsigned long vaddr, unsigned long n_pages)
 {