powerpc/boot: Fix CONFIG_PPC_MPC52XX references

Commit 866bfc75f40e ("powerpc: conditionally compile platform-specific
serial drivers") made some code depend on CONFIG_PPC_MPC52XX, which
doesn't exist.

Fix it to use CONFIG_PPC_MPC52xx.

Fixes: 866bfc75f40e ("powerpc: conditionally compile platform-specific serial drivers")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200724131728.1643966-7-mpe@ellerman.id.au
diff --git a/arch/powerpc/boot/Makefile b/arch/powerpc/boot/Makefile
index 4d43cb5..44af7154 100644
--- a/arch/powerpc/boot/Makefile
+++ b/arch/powerpc/boot/Makefile
@@ -117,7 +117,7 @@
 		elf_util.c $(zlib-y) devtree.c stdlib.c \
 		oflib.c ofconsole.c cuboot.c
 
-src-wlib-$(CONFIG_PPC_MPC52XX) += mpc52xx-psc.c
+src-wlib-$(CONFIG_PPC_MPC52xx) += mpc52xx-psc.c
 src-wlib-$(CONFIG_PPC64_BOOT_WRAPPER) += opal-calls.S opal.c
 ifndef CONFIG_PPC64_BOOT_WRAPPER
 src-wlib-y += crtsavres.S