ARM: Kirkwood: Fix 'SZ_1M' undeclared here for db88f6281-bp-setup.c

Linux-next has failed to compile for kirkwood since 23 August with:

arch/arm/mach-kirkwood/db88f6281-bp-setup.c:29: error: 'SZ_1M' undeclared here (not in a function)
arch/arm/mach-kirkwood/db88f6281-bp-setup.c:33: error: 'SZ_4M' undeclared here (not in a function)

Add missing <linux/sizes.h>

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>