riscv: dts: thead: Add TH1520 mmc controllers and sdhci clock

riscv: dts: thead: Add TH1520 mmc controllers and sdhci clock

Add node for the fixed reference clock used for emmc and sdio nodes.
Add emmc node for the 1st dwcmshc instance which is typically connected
to an eMMC device. Add sdio0 node for the 2nd dwcmshc instance which is
typically connected to microSD slot. Add sdio1 node for the 3rd dwcmshc
instance which is typically connected to an SDIO WiFi module. The node
names are based on Table 1-2 C910/C906 memory map in the TH1520 System
User Manual.

Link: https://git.beagleboard.org/beaglev-ahead/beaglev-ahead/-/tree/main/docs
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>