microbit-v2: add support for S113 SoftDevice

This currently doesn't work with `tinygo flash` yet (even with
`-programmer=openocd`), you can use pyocd instead. For example, from the
Bluetooth package:

    tinygo build -o test.hex -target=microbit-v2-s113v7 ./examples/advertisement/
    pyocd flash --target=nrf52 test.hex

I intend to add support for pyocd to work around this issue, so that a simple
`tinygo flash` suffices.

targets/microbit-v2-s113v7.json

@@ -0,0 +1,3 @@
+{
+	"inherits": ["microbit-v2", "nrf52833-s113v7"]
+}

targets/nrf52833-s113v7.json

@@ -0,0 +1,7 @@
+{
+	"build-tags": ["softdevice", "s113v7"],
+	"linkerscript": "targets/nrf52833-s113v7.ld",
+	"ldflags": [
+		"--defsym=__softdevice_stack=0x700"
+	]
+}

targets/nrf52833-s113v7.ld

@@ -0,0 +1,14 @@
+
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0x00000000 + 0x1C000, LENGTH = 0x80000 - 0x1C000
+    RAM (xrw)       : ORIGIN = 0x20000000 + 0x1e20,  LENGTH = 0x20000 - 0x1e20
+}
+
+_stack_size = 4K + __softdevice_stack;
+
+/* These values are needed for the Nordic SoftDevice. */
+__app_ram_base = ORIGIN(RAM);
+__softdevice_stack = DEFINED(__softdevice_stack) ? __softdevice_stack : 0;
+
+INCLUDE "targets/arm.ld"