feat: Add support for QEMU board and MMIO

- Added new file `qemu.rs` under `boards` module to define QEMU board specific constants
- Exported `MMIO` constant from `qemu.rs` to be used by other modules
- Updated `mod.rs` to include `qemu` module
- Added new constant `TRAMPOLINE` in `config.rs` to define the address of the trampoline function
- Updated `linker-qemu.ld` to include the `.text.trampoline` section
- Exposed `frame_dealloc` function in `frame.rs` to allow deallocation of physical frames