Add stub headers for buffer head management:
- buffer_head.h: buffer head structure and state management
functions (bh_*_bit helpers, BUFFER_FNS/TAS_BUFFER_FNS macros)
- journal-head.h: journal buffer head structure for jbd2
These provide minimal interfaces needed for ext4l filesystem
compilation in U-Boot.
Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
