arm64: Add simple earlyprintk support

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.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

arch/arm64/Kconfig.debug

@@ -24,4 +24,13 @@ config DEBUG_STACK_USAGE
 	  Enables the display of the minimum amount of free stack which each
 	  task has ever had available in the sysrq-T output.
 
+config EARLY_PRINTK
+	bool "Early printk support"
+	default y
+	help
+	  Say Y here if you want to have an early console using the
+	  earlyprintk=<name>[,<addr>][,<options>] kernel parameter. It
+	  is assumed that the early console device has been initialised
+	  by the boot loader prior to starting the Linux kernel.
+
 endmenu