dev/uart: Support setting the register io width

[andrew]

Summary

Some uarts require a specific register width. Support setting this in
the kernel environment.

Sponsored by: Arm Ltd

[imp]

With this, we can initialize all the fields in uart_bas. I like it, module a minor quibble.

sys/dev/uart/uart_subr.c

232

What should we initialize regiowidth to? Some drivers don't seem to initialize it at all.
The default is '1' :

static inline void
uart_setreg(struct uart_bas *bas, int reg, int value)
{

        switch (uart_regiowidth(bas)) {
        case 4:
                bus_space_write_4(bas->bst, bas->bsh, uart_regofs(bas, reg), value);
                break;
        case 2:
                bus_space_write_2(bas->bst, bas->bsh, uart_regofs(bas, reg), value);
                break;
        default:
                bus_space_write_1(bas->bst, bas->bsh, uart_regofs(bas, reg), value);
                break;
        }
}

but that breaks

sys/dev/uart/uart_cpu_acpi.c:		di->bas.regiowidth = 8;

silently :(.

tl;dr: I think we should explicitly initialize it here to 1. One of us should fix uart.h's uart_getreg and uart_setreg to have a 8 case (though that might be a landmine if we still have platforms without bus_space_write_8, but I thought that was a 'base' requirement of 32-bit platforms these days).

[andrew]

Set di->bas.regiowidth