This adds an early polled-mode "uart" console driver, based on Andi
Kleen's early_printk work.
The difference is that this locates the UART device directly by its
MMIO or I/O port address, so we don't have to make assumptions about
how ttyS devices will be named. After the normal serial driver
starts, we try to locate the matching ttyS device and start a console
there.
Sample usage:
console=uart,io,0x3f8
console=uart,mmio,0xff5e0000,115200n8
If the baud rate isn't specified, we peek at the UART to figure it
out.