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>
624e051372