ARM: omap: move board specific NAND configs out from ti_armv7_common.h

ARM: omap: move board specific NAND configs out from ti_armv7_common.h

This patch moves some board specific NAND configs:
- FROM: generic config file 'ti_armv7_common.h'
- TO:   individual board config files using these configs.
So that each board can independently set the value as per its design.

Following configs are affected in this patch:
  CONFIG_SYS_NAND_U_BOOT_OFFS: <refer doc/README.nand>
  CONFIG_CMD_SPL_NAND_OFS: <refer doc/README.falcon>
  CONFIG_SYS_NAND_SPL_KERNEL_OFFS: <refer doc/README.falcon>
  CONFIG_CMD_SPL_WRITE_SIZE: <refer doc/README.falcon>

This patch also updates documentation for few of above NAND configs.

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