ARM: rmobile: Add JTAG recovery support for M2 Porter

ARM: rmobile: Add JTAG recovery support for M2 Porter

Add JTAG recovery support into the M2 Porter TPL. This allows the
TPL to be loaded over JTAG, initialize the system, wait for the
JTAG debugger to load U-Boot image into RAM and then resume and
start U-Boot from RAM.

The procedure is as follows:
1) Load u-boot-tpl.bin to 0xe6300000
2) Write magic number 0x1337c0de to 0xe6300020
   TPL checks for this particular magic and starts JTAG recovery
   if this number is present. This is not present by default.
3) Start U-Boot TPL from 0xe6300000
4) Wait for a message from TPL on UART indicating JTAG boot:
   "JTAG boot detected!"
5) Halt the system in JTAG debugger
6) Load U-Boot image (u-boot.img) to 0x4fffffc0
7) Write magic number 0xb33fc0de to 0xe6300024
   TPL checks for this particular magic to verify that the U-Boot
   image was loaded into DRAM by the JTAG debugger.
8) Resume the system in JTAG debugger

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>