riscv: Skip riscv_cpu_setup() when CPU driver is disabled

Building on commit c64fc632a8 ("riscv: cpu: Use CONFIG_IS_ENABLED(CPU)
instead of plain ifdef"), add an early return in riscv_cpu_setup() when
CONFIG_CPU is not enabled. This allows platforms to save code space in
SPL by disabling CONFIG_SPL_CPU.

Without this patch, building U-Boot with CONFIG_CPU=n and CONFIG_EVENT=y
is broken: riscv_cpu_setup() is registered as an EVT_DM_POST_INIT_F event
spy, and when the CPU uclass is unavailable uclass_find_first_device()
returns no device, so the function returns -ENODEV. That in turn makes
event_notify_null() in dm_init_and_scan() fail and triggers a boot hang
("initcall initf_dm() failed"). Returning 0 early avoids that failure.

The compiler's dead-code elimination combined with --gc-sections
removes the unreachable code and all associated static data,
achieving significant size reduction without preprocessor guards:

  spl/u-boot-spl:all -4332 spl/u-boot-spl:rodata -2872
  spl/u-boot-spl:text -1460

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Yao Zi <me@ziyao.cc>

arch/riscv/cpu/cpu.c

@@ -638,6 +638,9 @@ int riscv_cpu_setup(void)
 	const char *isa, **exts;
 	struct udevice *dev;
 
+	if (!CONFIG_IS_ENABLED(CPU))
+		return 0;
+
 	uclass_find_first_device(UCLASS_CPU, &dev);
 	if (!dev) {
 		debug("unable to find the RISC-V cpu device\n");