sdh: add example for initializing and reading sdcard

[jakezhu9]

Background

Previously, we used the SPI interface to implement SD card read/write functionality. While this approach was effective, it required additional pin connections(see https://github.com/rustsbi/bouffalo-hal/blob/main/examples/peripherals/sdcard-demo/README.md) and had relatively slow data transfer speeds. To address these issues, we are now focusing on utilizing the SDH peripheral, which offers a more efficient and streamlined way to interact with SD cards.

Progress

We have developed a simple example that demonstrates the initialization of the SDH peripheral and reading data from an SD card. This example has been tested on the target hardware and is functioning correctly.

However, the current implementation is a basic demo intended for validation purposes. It relies on direct peripheral register access. In the future, we plan to improve the code structure, and support more advanced SD card operations.

[luojia65]

We can follow on to rearrange the code later. LGTM, excellent!