clk: rockchip: rk3588: Set SPLL frequency during SPL stage

All parts expect the SPLL to run at 702MHz. In U-Boot it's the SPLL_HZ
declaring this rate and in the kernel it's a fixed clock definition.

While everything is expecting 702MHz, the SPLL is not running that
frequency when coming from the bootrom though, instead it's running
at 351MHz and the vendor-u-boot just sets it to the expected frequency.

The SPLL itself is located inside the secure-BUSCRU and in theory
accessible as an SCMI clock, though this requires an unknown amount
of cooperation from trusted-firmware to set at a later stage, though
during the SPL stage we can still access the relevant CRU directly.

The SPLL is for example necessary for the DSI controllers to produce
output.

As the SPLL is "just" another rk3588 pll, just set the desired rate
directly during the SPL stage.

Tested on rk3588-rock5b and rk3588-tiger by reading back the PLL rate
and also observing working DSI output with this change.

Fixes: 67377716 ("rockchip: rk3588: Add support for sdmmc clocks in SPL")
Suggested-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@cherry.de>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>