mb/asus/p8x7x-series: Revert to native max_mem_clock_mhz of 800

The setting was reduced to 666 for native raminit in commit
7039edd2da (SNB+MRC boards: Migrate MRC settings to
devicetree) based on boot test results at the time.

With more changes merged, additional native raminit tests were
done on p8z77-m. It is now possible for previously failing
memory configurations to operate at full speed. This, combined
with multiple reports on gerrit that this family does work at
800, warrants returning the setting to what it was.

Change-Id: I1fbe9c8d076fcd633f71424d60585681c40677c4
Signed-off-by: Keith Hui <buurin@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79726
Reviewed-by: Matt DeVillier <matt.devillier@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asus/p8x7x-series/devicetree.cb

@@ -4,8 +4,7 @@ chip northbridge/intel/sandybridge
 	# All MRC-capable boards in family (P8Z77-M[ PRO]) lists supported
 	# DIMMs down to 1.25v
 	register "ddr3lv_support" = "1"
-	# FIXME: Nothing can run native at 800MHz on p8z77-m, others may have same problem
-	register "max_mem_clock_mhz" = "CONFIG(USE_NATIVE_RAMINIT) ? 666 : 800"
+	register "max_mem_clock_mhz" = "800"
 	register "spd_addresses" = "{0x50, 0x51, 0x52, 0x53}"
 
 	register "usb_port_config" = "{