MIPS: BCM63xx: Sync MIPS counters during CPU bringup

We are using the mips counters as the clock source, so we need to ensure
they are synced, else e.g. gettimeofday will return different values
depending on which core it was run.

Observed difference was about 8 seconds, causing ~8 seconds ping or time
running backwards for some programs.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/7265/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -139,6 +139,7 @@ config BCM63XX
 	select BOOT_RAW
 	select CEVT_R4K
 	select CSRC_R4K
+	select SYNC_R4K
 	select DMA_NONCOHERENT
 	select IRQ_CPU
 	select SYS_SUPPORTS_32BIT_KERNEL