arch/riscv/Kconfig: Make correct default value for CONFIG_ARCH_RISCV_M

Change-Id: Ib9329904060cab48d527de1b1ccdab5b6fe71b99
Signed-off-by: Subrata Banik <subrata.banik@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33144
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/riscv/Kconfig

@@ -24,8 +24,8 @@ config ARCH_RISCV_M
 	# one implementation that will not have it due
 	# to security concerns.
 	bool
-	default n if ARCH_RISCV_M_DISABLED
-	default y
+	default y if ARCH_RISCV && !ARCH_RISCV_M_DISABLED
+	default n
 
 config ARCH_RISCV_S
 	# S (supervisor) mode is for kernels. It is optional.