ARM: dts: am335x-evm: Fix 5th NAND partition's name

The 5th NAND partition should be named "NAND.u-boot-spl-os"
instead of "NAND.u-boot-spl". This is to be consistent with other
TI boards as well as u-boot.

Fixes: 91994fac

 ("ARM: dts: am335x-evm: NAND: update MTD partition table")

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>