arm64: dts: rockchip: Fix sdmmc access on rk3308-rock-s0 v1.1 boards

arm64: dts: rockchip: Fix sdmmc access on rk3308-rock-s0 v1.1 boards

BootROM leave GPIO4_D6 configured as SDMMC_PWREN function and DW MCI
driver set PRWEN high on MMC_POWER_UP and low on MMC_POWER_OFF.
Similarly U-Boot also set PRWEN high before accessing mmc.

However, HW revision prior to v1.2 must pull GPIO4_D6 low to access
sdmmc. For HW revision v1.2 the state of GPIO4_D6 has no impact.

Model an always-on active low fixed regulator using GPIO4_D6 to fix
use of sdmmc on older HW revisions of the board.

Fixes: adeb5d2a4ba4 ("arm64: dts: rockchip: Add Radxa ROCK S0")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Link: https://lore.kernel.org/r/20241119230838.4137130-1-jonas@kwiboo.se
Signed-off-by: Heiko Stuebner <heiko@sntech.de>