serial_core: support native endianness

There are three natural ways in which devices may be wired to the system:
little endian (device receives correctly ordered bits of a word written
by little-endian CPU to its register, but big-endian CPU needs to swap
bytes of a word before writing it), big endian (same, but with big-endian
CPU in more favourable position) and native endian (CPU of either
endianness may do word-sized I/O without need for byteswapping).

Adding an option for native endianness allows using single kernel command
line for boards with native-endian serial ports on bi-endian
architectures. This goes in parallel with 'native-endian' DTS attribute.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -1819,8 +1819,8 @@ uart_get_console(struct uart_port *ports, int nr, struct console *co)
  *	@options: ptr for <options> field; NULL if not present (out)
  *
  *	Decodes earlycon kernel command line parameters of the form
- *	   earlycon=<name>,io|mmio|mmio32|mmio32be,<addr>,<options>
- *	   console=<name>,io|mmio|mmio32|mmio32be,<addr>,<options>
+ *	   earlycon=<name>,io|mmio|mmio32|mmio32be|mmio32native,<addr>,<options>
+ *	   console=<name>,io|mmio|mmio32|mmio32be|mmio32native,<addr>,<options>
  *
  *	The optional form
  *	   earlycon=<name>,0x<addr>,<options>
@@ -1841,6 +1841,10 @@ int uart_parse_earlycon(char *p, unsigned char *iotype, unsigned long *addr,
 	} else if (strncmp(p, "mmio32be,", 9) == 0) {
 		*iotype = UPIO_MEM32BE;
 		p += 9;
+	} else if (strncmp(p, "mmio32native,", 13) == 0) {
+		*iotype = IS_ENABLED(CONFIG_CPU_BIG_ENDIAN) ?
+			UPIO_MEM32BE : UPIO_MEM32;
+		p += 13;
 	} else if (strncmp(p, "io,", 3) == 0) {
 		*iotype = UPIO_PORT;
 		p += 3;