The cpu mask flag value should match the number of cpu cores in the
chip. Correct the value to two cpus for BCM6878 dual core SoC.

Fixes: 6bcad714 ("ARM: dts: Add DTS files for bcmbca SoC BCM6878")
Signed-off-by: William Zhang <william.zhang@broadcom.com>
Link: https://lore.kernel.org/r/20220801194754.29492-1-william.zhang@broadcom.comSigned-off-by: Florian Fainelli <f.fainelli@gmail.com>