This patch adds support for "earlyprintk=" parameter on the kernel
command line. The format is:
earlyprintk=<name>[,<addr>][,<options>]
where <name> is the name of the (UART) device, e.g. "pl011", <addr> is
the I/O address. The <options> aren't currently used.
The mapping of the earlyprintk device is done very early during kernel
boot and there are restrictions on which functions it can call. A
special early_io_map() function is added which creates the mapping from
the pre-defined EARLY_IOBASE to the device I/O address passed via the
kernel parameter. The pgd entry corresponding to EARLY_IOBASE is
pre-populated in head.S during kernel boot.
Only PL011 is currently supported and it is assumed that the interface
is already initialised by the boot loader before the kernel is started.