ARM: shmobile: dts: koelsch: Fix flash partition label and size

Update the size and names of flash partitions to match the
expectations of the loader which are as follows:

"loader"---0x0000_0000-0x0008_0000 [loader program (readonly)]
"user"  ---0x0008_0000-0x0060_0000 [U-Boot + bootargs + dt + uImage (readonly)]
"flash" ---0x0060_0000-0x0400_0000 [filesystem and free (read/write)]

["user"'s assumed breakdown]
U-boot       (0x0008_0000-0x000c_0000) 256KiB
bootargs     (0x000c_0000-0x0010_0000) 256KiB
Device tree  (0x0010_0000-0x0014_0000) 256KiB
zImage       (0x0014_0000-0x0060_0000) 4.75MiB

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>