board: rzg2l: Add RZ/G2L SMARC EVK board

The Renesas RZ/G2L SMARC Evaluation Board Kit consists of the RZ/G2L
System-on-Module (SOM) based on the R9A07G044L2 SoC, and a common SMARC
carrier board.

The ARM TrustedFirmware code for the Renesas RZ/G2L SoC family passes a
devicetree blob to the bootloader as an argument in the same was
previous R-Car gen3/gen4 SoCs. This blob contains a compatible string
which can be used to identify the particular SoC we are running on and
this is used to select the appropriate device tree to load.

The configuration renesas_rzg2l_smarc_defconfig is added to support
building for this target. In the future this defconfig will be extended
to support other SoCs and evaluation boards from the RZ/G2L family.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>