powerpc/83xx: Fix build failure with FPU=n

Building eg. 83xx/mpc832x_rdb_defconfig with FPU=n, fails with:

arch/powerpc/platforms/83xx/suspend.c: In function 'mpc83xx_suspend_enter':
arch/powerpc/platforms/83xx/suspend.c:209:17: error: implicit declaration of function 'enable_kernel_fp'
209 | enable_kernel_fp();

Fix it by providing an enable_kernel_fp() stub for FPU=n builds,
which allows using IS_ENABLED(CONFIG_PPC_FPU) around the call to
enable_kernel_fp().

Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/include/asm/switch_to.h

···
 #else
 static inline void save_fpu(struct task_struct *t) { }
 static inline void flush_fp_to_thread(struct task_struct *t) { }
+static inline void enable_kernel_fp(void)
+{
+	BUILD_BUG();
+}
 #endif
 
 #ifdef CONFIG_ALTIVEC

arch/powerpc/platforms/83xx/suspend.c

···
 		out_be32(&pmc_regs->config1,
 		         in_be32(&pmc_regs->config1) | PMCCR1_POWER_OFF);
 
-		enable_kernel_fp();
+		if (IS_ENABLED(CONFIG_PPC_FPU))
+			enable_kernel_fp();
 
 		mpc83xx_enter_deep_sleep(immrbase);