MIPS: Malta: use generic 8250 early console

This patch switches Malta from using the MIPS implementation of early
printk with Malta's prom_putchar to using the generic 8250_early
implementation. This offers a couple of advantages:

  - We duplicate less generic code.

  - The UART can be initialised rather than being reliant upon
    inheriting a valid setup from the bootloader.

The Malta console_config function is extended to initialise the early
console if no earlycon= kernel parameter is provided, inheriting the
modetty0 bootloader environment if present and falling back to a
default 38400n8r setup if not. This matches the behaviour used for the
regular console= parameter.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6183/

arch/mips/Kconfig

@@ -320,7 +320,6 @@ config MIPS_MALTA
 	select SYS_HAS_CPU_MIPS64_R2
 	select SYS_HAS_CPU_NEVADA
 	select SYS_HAS_CPU_RM7000
-	select SYS_HAS_EARLY_PRINTK
 	select SYS_SUPPORTS_32BIT_KERNEL
 	select SYS_SUPPORTS_64BIT_KERNEL
 	select SYS_SUPPORTS_BIG_ENDIAN