it obtained through the uart_class structure. This allows us
to declare the uart_class structure as weak and as such allows
us to reference it even when it's not compiled-in.
It also allows is to get the uart_ops structure by name, which
makes it possible to implement the dt tag handling in uart_getenv().
The side-effect of all this is that we're using the uart_class
structure more consistently which means that we now also have
access to the size of the bus space block needed by the hardware
when we map the bus space, eliminating any hardcoding.

  that can be used to check whether receive data is ready, i.e. whether
  the subsequent call of uart_poll() should return a char, and unlike
  uart_poll() doesn't actually receive data.
- Remove the device-specific implementations of uart_poll() and implement
  uart_poll() in terms of uart_getc() and the newly added uart_rxready()
  in order to minimize code duplication.
- In sunkbd(4) take advantage of uart_rxready() and use it to implement
  the polled mode part of sunkbd_check() so we don't need to buffer a
  potentially read char in the softc.
- Fix some mis-indentation in sunkbd_read_char().

Discussed with:	marcel

quick hack to deal with this. We may need to formalize this better
and have this information come from the bus attachments.

on PowerPC use the current setting of the hardware and not second
guess what the OFW does on various machines.

   divisor. This allows us to set the line speed to the maximum
   of 1/4 of the device clock.
o  Disable the baudrate generator before programming the line
   settings, including baudrate, and enable it afterwards.

This happens when the baudrate is too high for the given RCLK.

for a character to be received. Instead let getc() do any necesary
locking.

Stop using our local UART_IPEND_* and instead use the global SER_INT_*
as defined in <sys/serial.h>.