MIPS: BCM63xx: Fix soft-reset lockup on BCM6345

This patch fixes a lockup on BCM6345 where setting the PLL soft reset bit
will also lock the other blocks including UART.  Instead of setting only
the PLL soft reset bit in the software reset register, set this bit but do
not touch the others.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>