[PATCH v1 1/1] x86/early_printk: Use 'mmio32' for consistency

[Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>]

First of all, using 'mmio' prevents proper implementation of 8-bit accessors.
Second, it's simply inconsistent with uart8250 set of options. Rename it to
'mmio32'. While at it, remove rather misleading comment in the documentation.