linker: devicetree_regions: Add support memory region flag setting

Add `zephyr,memory-region-flags` for supporting memory region flags
setting.

For example, when the below node is in the devicetree,

```
    test_sram: sram@20010000 {
        compatible = "zephyr,memory-region", "mmio-sram";
        reg = < 0x20010000 0x1000 >;
        zephyr,memory-region = "FOOBAR";
        zephyr,memory-region-flags = "rw";
    };
```

We get the following line in MEMORY section of linker script.

```
FOOBAR (rw) : ORIGIN = (0x20010000), LENGTH = (0x1000)
```

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>