arm: dts: socfpga: agilex5: Add dedicated eMMC device tree support

arm: dts: socfpga: agilex5: Add dedicated eMMC device tree support

Add dedicated device tree support for eMMC configuration on the Agilex5
SoCDK board, providing an alternative to the default SD card setup.

Changes to socfpga_agilex5.dtsi:
-
- Configure always-on regulator for stable eMMC operation

New device tree files:
- socfpga_agilex5_socdk_emmc.dts: Main eMMC device tree configuration
  * Configure for eMMC operation (no-sd, no-sdio, non-removable)
  * Set 8-bit bus width and high speed capability
  * Add timing parameters for legacy and SDR modes
  * Configure voltage supplies for eMMC power and I/O
  * Add fixed 1.8V regulator for eMMC I/O voltage supply

- socfpga_agilex5_socdk_emmc-u-boot.dtsi: U-Boot specific additions
  * Include common Agilex5 U-Boot configurations
  * Set SPL boot order with eMMC support
  * Enable necessary peripherals for boot-time operation

Configuration files:
- configs/socfpga_agilex5_emmc_defconfig: eMMC-specific configuration
  * Inherit from base Agilex5 configuration
  * Disable GPIO regulator support (not needed for fixed eMMC setup)
  * Set eMMC-specific device tree

Build system integration:
- Add socfpga_agilex5_socdk_emmc.dtb target to Makefile

Signed-off-by: Tanmay Kathpalia <tanmay.kathpalia@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>