m53menlo: Switch to deriving CONFIG_SYS_NAND_PAGE_COUNT

Typically platforms will define CONFIG_SYS_NAND_PAGE_COUNT based on
CONFIG_SYS_NAND_BLOCK_SIZE / CONFIG_SYS_NAND_PAGE_SIZE.  Switch to this
in preparation for migrating CONFIG_SYS_NAND namespace to Kconfig.

Cc: Marek Vasut <marex@denx.de>
Cc: Olaf Mandel <o.mandel@menlosystems.com>
Signed-off-by: Tom Rini <trini@konsulko.com>