arch/riscv: Makefile.mk: Fix incorrect config variable

ARCH_RISCV_PMP should be CONFIG_ARCH_RISCV_PMP. Rename it.

Change-Id: I2a22acae5cd9f30e01c491653bf7fc7b7765d815
Signed-off-by: Ronald G Minnich <rminnich@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/81086
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/src/arch/riscv/Makefile.mk b/src/arch/riscv/Makefile.mk
index 0dbfd2a..1131bd5 100644
--- a/src/arch/riscv/Makefile.mk
+++ b/src/arch/riscv/Makefile.mk
@@ -59,7 +59,7 @@
 all-y += boot.c
 all-y += smp.c
 all-y += misc.c
-all-$(ARCH_RISCV_PMP) += pmp.c
+all-$(CONFIG_ARCH_RISCV_PMP) += pmp.c
 all-y += \
 	$(top)/src/lib/memchr.c \
 	$(top)/src/lib/memcmp.c \