ARM: 8638/1: mtd: lart: Rename partition defines to be prefixed with PART_

In preparation for defining KERNEL_START on ARM, rename KERNEL_START to
PART_KERNEL_START, and to be consistent, do this for all
partition-related constants.

Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>