platforms/targets: Add Milianke MLK-CU07-KU15P support

[Kingfish404]

Add the platform and reference LiteX target for the Milianke MLK-H5P-CU07-KU15P development board.

Define the system and DDR reference clocks, UART, SD card, user LEDs and buttons, I2C, and the board's 4 GB 64-bit DDR4 interface.

Add a target using the UltraScale+ DDR PHY, deterministic power-on reset, optional SD card support, and an optional LED chaser.

Configure OpenFPGALoader and SPI x4 bitstream generation for the XCKU15P-FFVA1156-2-E device.

Already tested on a real hardware: https://x.com/kingfish404/status/2080158498843951387.

[enjoy-digital]

Thanks @Kingfish404! I'm reviewing and will merged it once done.

[enjoy-digital]

Reviewed and pushed follow-up commit 095e9fa.

Changes:

• aligned the platform/target layout, argument tables, keyword assignments, and comments with the surrounding LiteX-Boards style;
• selected the UltraScale+ USPMMCM/USPIDELAYCTRL helpers for the XCKU15P, producing the matching MMCME4_ADV primitive;
• made the standalone power-on-reset clock domain explicitly reset-less while preserving the reset policy;
• regenerated the README board list, which was the missing CI-generated update.

Validation:

• all six CI guardrail scripts pass;
• parser/style/board tests pass (14 tests);
• default target, SDCard + LED chaser, integrated main RAM, and generic platform builds all pass with --no-compile.

I did not find another issue in the reviewed platform/target integration.

[enjoy-digital]

@Kingfish404: This is merged, thanks again!