FIX: fix vcu118 sd card frequency

[T-K-233]

The Chisel code is using MHz as the frequency base unit, while the C code is using Hz, so we need to convert between these two units to get the correct SPI clock frequency to drive the SD card.

The SPI clock frequency is set to "High Speed" mode (25 MHz) [1] [2], which should be supported by all modern SD cards. If the clock is not working, user should try to decrease to normal mode (12.5 MHz) by changing the macro definition in sd.c:

before:

#deifine SPI_CLK      25000

after:

#deifine SPI_CLK      12500

The High Speed clock is tested on the SanDisk Ultra 32G HC I SD card.

Reference
[1] https://www.cactus-tech.com/wp-content/uploads/2019/03/An-Introduction-To-SD-Card-Interface.pdf
[2] https://en.wikipedia.org/wiki/SD_card#:~:text=Supports%20a%20clock%20frequency%20of,could%20transfer%20104%20MB%2Fs.

Related PRs / Issues:

Type of change:

• Bug fix
• New feature
• Other enhancement

Impact:

• RTL change
• Software change (RISC-V software)
• Build system change
• Other

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[jerryz123]

I don't understand the change/hack, but this is fine.

[T-K-233]

I don't understand the change/hack, but this is fine.

will add more information after I fully test it