feat: add PSRAM-DMA flag + runtime API for ESP32-S2/S3; remove 16MHz heuristic

[RubenKelevra]

Description

Add a Kconfig switch (CONFIG_CAMERA_PSRAM_DMA) to enable camera to PSRAM
DMA copy on ESP32-S2 and ESP32-S3 targets, in favor of removing the
undocumented "set XCLK=16MHz to enable PSRAM DMA" heuristic.

cam_hal.c:

• Select psram_mode = true when CONFIG_CAMERA_PSRAM_DMA && target is S2/S3.
• Otherwise psram_mode = false.
• Log whether PSRAM DMA mode is enabled.

Kconfig:

• New bool CAMERA_PSRAM_DMA (default n) under Camera configuration.

Runtime control:

• New public API:
  • esp_camera_set_psram_mode(bool enable)
  • esp_camera_get_psram_mode(void)
  • esp_camera_reconfigure(const camera_config_t *cfg)

README/test:

• Remove stale 16MHz XCLK comments.
• Add short note describing CONFIG_CAMERA_PSRAM_DMA.

Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[me-no-dev]

please think of a way to make this runtime configurable. The driver is used in Arduino, which uses a pre-compiled version of the driver and can not change sdkconfig on the fly

[RubenKelevra]

Oh okay, wasn't aware of this limitation.

What do you think of putting the capability behind a build flag thought, that would make the binary a bit smaller on devices which don't need it (which do usually have also very limited ROM).

[me-no-dev]

The driver runs only on ESP32, ESP32S2 and ESP32S3 which do not have such limitation. One option is to have a flag in the camera config (at the end), but preferably is to be set some other way, like a method that would set a flag in the driver, so it does not break existing code.

[RubenKelevra]

Okay, will do, as soon as I'm back home.

Since you've implemented it: Have you recently tested this mode?