arm64: dts: renesas: rzv2-evk-cn15-sd: Add ramp delay for SD0 regulator

arm64: dts: renesas: rzv2-evk-cn15-sd: Add ramp delay for SD0 regulator

Set an appropriate ramp delay for the SD0 I/O voltage regulator in the
CN15 SD overlay to make UHS-I voltage switching reliable during card
initialization.

This issue was observed on the RZ/V2H EVK, while the same UHS-I cards
worked on the RZ/V2N EVK without problems. Adding the ramp delay makes
the behavior consistent and avoids SD init timeouts.

Before this change SD0 could fail with:

    mmc0: error -110 whilst initialising SD card

With the delay in place UHS-I cards enumerate correctly:

    mmc0: new UHS-I speed SDR104 SDXC card at address aaaa
    mmcblk0: mmc0:aaaa SR64G 59.5 GiB
     mmcblk0: p1

Fixes: 3d6c2bc7629c8 ("arm64: dts: renesas: Add CN15 eMMC and SD overlays for RZ/V2H and RZ/V2N EVKs")
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20260123225957.1007089-5-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>