arm64: dts: qcom: Add base qrb4210-rb2 board dts

Add DTS for Qualcomm qrb4210-rb2 board which uses SM4250 SoC.

This adds debug uart, emmc, uSD and tlmm support along with
regulators found on this board.

Also defines the 'xo_board' and 'sleep_clk' frequencies for
this board.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
[bjorn: Squashed separate patch specifying regulator-system-load]
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230411072840.2751813-3-bhupesh.sharma@linaro.org