arm: mediatek: dts: activate SMP for mt6589

This simple patch activates SMP for mt6589 by adding the missing
"enable-method" property. After applying this patch kernel log
indicates all cores are brought up:

[    0.070122] CPU0: thread -1, cpu 0, socket 0, mpidr 80000000
[    0.071652] Setting up static identity map for 0x80100000 - 0x80100054
[    0.072711] rcu: Hierarchical SRCU implementation.
[    0.073853] smp: Bringing up secondary CPUs ...
[    0.133675] CPU1: thread -1, cpu 1, socket 0, mpidr 80000001
[    0.193675] CPU2: thread -1, cpu 2, socket 0, mpidr 80000002
[    0.253675] CPU3: thread -1, cpu 3, socket 0, mpidr 80000003
[    0.253818] smp: Brought up 1 node, 4 CPUs
[    0.256930] SMP: Total of 4 processors activated (7982.28 BogoMIPS).
[    0.257855] CPU: All CPU(s) started in SVC mode.

Before this change CPU cores 1-3 didn't start and the following lines
were in kernel log:

[    0.070126] CPU0: thread -1, cpu 0, socket 0, mpidr 80000000
[    0.071640] Setting up static identity map for 0x80100000 - 0x80100054
[    0.072706] rcu: Hierarchical SRCU implementation.
[    0.073850] smp: Bringing up secondary CPUs ...
[    0.076052] smp: Brought up 1 node, 1 CPU
[    0.076678] SMP: Total of 1 processors activated (2000.48 BogoMIPS).
[    0.077603] CPU: All CPU(s) started in SVC mode.

Signed-off-by: Boris Lysov <arzamas-16@mail.ee>
Link: https://lore.kernel.org/r/20210314023735.052d2d35@pc


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>