board/ti/am335x: update configs for parallel NAND

This patch
- consolidate CONFIG_SYS_NAND_xx and CONFIG_SPL_NAND_xx from various
  configuration files into single file.
- update MTD Partition table to match AM335x_EVM DT in linux-kernel
- segregate CONFIGs based on different boot modes (like SPL and U-Boot)

Signed-off-by: Pekon Gupta <pekon@ti.com>