pcm052: fix MTD partitioning

Merge 'spare' into 'bootloader' partition
Use same partition for ramdisk and rootfs boot scenarios.
Remove 'ramdisk' partition, use 'rootfs' for ramdisk
(ramdisk and nand boot scenarios are mutually exclusive).
Expand last partition to end of actual NAND size.
Adjust UBIFS rootfs boot kernel arguments.

Signed-off-by: Albert ARIBAUD (3ADEV) <albert.aribaud@3adev.fr>