Move .rodata in Corstone-300 linker script

Move *(.rodata) and *(.rodata.*) from ITCM to DTCM (exec region) and use
the DRAM as load region, to avoid the ITCM memory from overflowing when
building the message_handler_openamp application in particular, using
gcc. CMSIS startup code will copy the data from DRAM to DTCM using the
added copy_table entry.

The message_handler_openamp application contains fallback support for
all TFLM operators (using a "MicroMutableAllOpResolver"), which is
causing the firmware size to get quite big and go past the memory size
of ITCM (512KiB) for Corstone-300.

Change-Id: I521ae3e417c65387bde3aa89e005580502823bef
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>