intel/skylake: Work around ROMCC optimization bug

On Skylake systems, the bootblock fails to compile with the following
error message:

bootblock_simple.c:6.1:
0x13930e0 copy       Internal compiler error: non dominated rhs use point
0x13a3f70?
Aborted (core dumped)

The option -fno-simplify-phi works around the issue, but will cause the
code to use more registers, hence we also need to enable -mcpu=p4 (see
intel/truxton mainboard for another example of where this has been done
in the past)

Change-Id: Iea1a1ba18d76c7323bb626c5f4b0032e4ee04a86
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: https://review.coreboot.org/12719
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/soc/intel/skylake/Makefile.inc

@@ -78,4 +78,6 @@ CPPFLAGS_common += -I$(src)/vendorcode/intel/fsp/fsp1_1/skylake
 # Currently used for microcode path.
 CPPFLAGS_common += -I3rdparty/blobs/mainboard/$(CONFIG_MAINBOARD_DIR)
 
+ROMCCFLAGS := -mcpu=p4 -fno-simplify-phi -O2
+
 endif