riscv: fix non-SMP support

Use CONFIG_CPU_MAX which defaults to 1 instead of CONFIG_RISCV_HART_NUM.
The default value of CONFIG_RISCV_HART_NUM was 0 and cause a jump to address 0.
Add a die() call to fail gracefully.

Change-Id: I4e3aa09b787ae0f26a4aae375f4e5fcd745a0a1e
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/c/29993
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Xiang Wang <wxjstz@126.com>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>

src/soc/sifive/fu540/Kconfig

@@ -39,7 +39,7 @@ config RISCV_CODEMODEL
 	string
 	default "medany"
 
-config RISCV_HART_NUM
+config MAX_CPUS
 	int
 	default 5