ARM: shmobile: rcar-gen2: Remove CMA reservation code

If CONFIG_HIGHMEM=y, two reserved blocks are allocated on R-Car Gen2:

    cma: Reserved 256 MiB at 0x70000000 on node -1
    cma: Reserved 64 MiB at 0x6c000000 on node -1

The first block is reserved by the family-specific rcar_gen2_reserve(),
the second by the common arm_memblock_init() (shmobile_defconfig sets
CONFIG_CMA_SIZE_MBYTES=64).  As both blocks are reserved (eventually)
using dma_contiguous_reserve_area(), they both have the same name
("reserved").  Hence if CONFIG_CMA_SYSFS=y:

  sysfs: cannot create duplicate filename '/kernel/mm/cma/reserved'
   ...
   cma_sysfs_init from do_one_initcall+0x84/0x178
   ...
  kobject: kobject_add_internal failed for reserved with -EEXIST, don't try to register things with the same name in the same directory.

This causes cma_sysfs_init() to fail completely, and not to create
/sys/kernel/mm/cma/ at all.

Fix this by dropping the R-Car Gen2-specific reservation.  Compared to
when it was introduced, now there exist more flexible mechanisms to
control the size of memory reserved for CMA.  Users can reserve more
memory by increasing CONFIG_CMA_SIZE_MBYTES, passing the cma=<N> kernel
command line parameter, or adding a reserved-memory/linux,cma node to
DT.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/3d38f4fec20c4af46e4570012de7017eee9a39e9.1736249109.git.geert+renesas@glider.be

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