drivers: flash: cc13xx_cc26xx: Add support of write to flash from flash

Currently, if we try to write data from a memory mapped flash address
to the flash then we get a -EINVAL error.
This could break the settings subsystem when we use NVS backend.

In order to make it work, add a buffer and use it when we want to
write data that is located in flash address space.

This has been tested using sample/subsys/settings/

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>