board: freescale: p1_p2_rdb_pc: Use named macros for i2c bus num and address

Replace hardcoded boot i2c bus num and address by existing macros when
generating env for CONFIG_EXTRA_ENV_SETTINGS.

Same macros are used in U-Boot board code when reading information from
boot i2c data.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>