powerpc/book3s64: fix link error with CONFIG_PPC_RADIX_MMU=n

Fix link error when CONFIG_PPC_RADIX_MMU is disabled:
powerpc64-linux-gnu-ld: arch/powerpc/platforms/pseries/lpar.o:(.toc+0x0): undefined reference to `mmu_pid_bits'
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200917020643.90375-1-yangyingliang@huawei.com

arch/powerpc/platforms/pseries/lpar.c

@@ -1724,6 +1724,7 @@ void __init hpte_init_pseries(void)
 		pseries_lpar_register_process_table(0, 0, 0);
 }
 
+#ifdef CONFIG_PPC_RADIX_MMU
 void radix_init_pseries(void)
 {
 	pr_info("Using radix MMU under hypervisor\n");
@@ -1731,6 +1732,7 @@ void radix_init_pseries(void)
 	pseries_lpar_register_process_table(__pa(process_tb),
 						0, PRTB_SIZE_SHIFT - 12);
 }
+#endif
 
 #ifdef CONFIG_PPC_SMLPAR
 #define CMO_FREE_HINT_DEFAULT 1