nand/fsl_elbc: detect page size at runtime

This avoids needing a separate U-Boot config when some revisions
of a board have small-page NAND and other revisions have large-page
NAND (except for NAND SPL targets).

CONFIG_FSL_ELBC_FMR is removed -- it was never used nor documented, and
it gets in the way of this change.

Signed-off-by: Scott Wood <scottwood@freescale.com>