lm32: todo and documentation

This patch adds general target documentation and a todo list.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>

target-lm32/README

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+LatticeMico32 target
+--------------------
+
+General
+-------
+All opcodes including the JUART CSRs are supported.
+
+
+JTAG UART
+---------
+JTAG UART is routed to a serial console device. For the current boards it
+is the second one. Ie to enable it in the qemu virtual console window use
+the following command line parameters:
+  -serial vc -serial vc
+This will make serial0 (the lm32_uart) and serial1 (the JTAG UART)
+available as virtual consoles.
+
+
+Programmatically terminate the emulator
+----------------------------------------
+Originally neither the LatticeMico32 nor its peripherals support a
+mechanism to shut down the machine. Emulation aware programs can write to a
+to a special register within the system control block to shut down the
+virtual machine.  For more details see hw/lm32_sys.c. The lm32-evr is the
+first BSP which instantiate this model. A (32 bit) write to 0xfff0000
+causes a vm shutdown.
+
+
+Special instructions
+--------------------
+The translation recognizes one special instruction to halt the cpu:
+  and r0, r0, r0
+On real hardware this instruction is a nop. It is not used by GCC and
+should (hopefully) not be used within hand-crafted assembly.
+Insert this instruction in your idle loop to reduce the cpu load on the
+host.
+
+
+Ignoring the MSB of the address bus
+-----------------------------------
+Some SoC ignores the MSB on the address bus. Thus creating a shadow memory
+area. As a general rule, 0x00000000-0x7fffffff is cached, whereas
+0x80000000-0xffffffff is not cached and used to access IO devices. This
+behaviour can be enabled with:
+  cpu_lm32_set_phys_msb_ignore(env, 1);
+

target-lm32/TODO

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+* disassembler (lm32-dis.c)
+* linux-user emulation
+* native bp/wp emulation (?)