ARM: dts: dra7-evm: Fix 8th NAND partition's name

The 8th NAND partition should be named "NAND.u-boot-env.backup1"
instead of "NAND.u-boot-env". This is to be consistent with other
TI boards as well as u-boot.

CC: Pekon Gupta <pekon@pek-sem.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>