microwatt probes serial ports twice

[shenki]

The microwatt machine calls of_platform_default_populate in an arch_initcall, which causes it to probe some drivers twice:

[    1.185779] Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled
[    1.212557] printk: console [ttyS0] disabled
[    1.245614] serial8250.0: ttyS0 at MMIO 0xc0002000 (irq = 16, base_baud = 6250000) is a 165A
[    1.275052] printk: console [ttyS0] enabled
[    1.283472] printk: bootconsole [udbg0] disabled
[    1.320930] printk: console [ttyS0] disabled
[    1.335603] c0002000.serial: ttyS0 at MMIO 0xc0002000 (irq = 16, base_baud = 6250000) is a 0
[    1.336024] printk: console [ttyS0] enabled

which doesn't cause any issues I've noticed, but it is unnecessary.

setup_arch calls find_legacy_serial_ports which causes the double registration. This function also does some of the console handling.

This is related to #404.