i2c: boardinfo: Annotate code used in init phase only

Annotate two places in boardinfo code:
- __i2c_first_dynamic_bus_num is set in init phase. Annotate it as
  __ro_after_init to prevent later changes.
- i2c_register_board_info() is used in init phase only, so annotate it
  as __init, allowing to free the memory after init phase.
  This is safe, see comment: "done in board-specific init code near
  arch_initcall() time"

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>