arm64: dts: renesas: Add CN15 eMMC and SD overlays for RZ/V2H and RZ/V2N EVKs

arm64: dts: renesas: Add CN15 eMMC and SD overlays for RZ/V2H and RZ/V2N EVKs

Introduce device tree overlays for supporting the eMMC (RTK0EF0186B02000BJ)
and microSD (RTK0EF0186B01000BJ) sub-boards connected via the CN15
connector on the RZ/V2H and RZ/V2N evaluation kits.

These overlays enable SDHI0 with appropriate pin control settings, power
regulators, and GPIO handling. Both sub-boards are supported using shared
overlay files that can be applied to either EVK due to their identical
connector layout and interface support.

To support this, new DT overlay files are added:
- `rzv2-evk-cn15-emmc.dtso` for eMMC
- `rzv2-evk-cn15-sd.dtso` for microSD

Additionally, the base DTS files for both EVKs are updated to include a
fixed 1.8V regulator (`reg_1p8v`) needed by the eMMC sub-board and
potential future use cases such as HDMI output.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/20250627193742.110818-1-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>