am335x_evm: am44xx_evm: dra7xx_evm: nand: Fix file-system partition name

We almost always use UBIFS for user accessible NAND file systems and
the UBIFS file system might contain more than one volume within the
single NAND partition. The last NAND partition is therefore more
appropriately named as "NAND.file-system" instead of "NAND.rootfs"

The Linux kernel (as of v3.16) also uses "NAND.file-system" to name the
last NAND partition. This patch makes the partition name consistent
between u-boot and the kernel.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>