mvebu: 5.15: fix compilation warning for mangle bootargs patch

mvebu: 5.15: fix compilation warning for mangle bootargs patch

Fix compilation warning for mangle bootargs patch.
Fix compilation warning:
arch/arm/boot/compressed/atags_to_fdt.c: In function 'append_rootblock':
arch/arm/boot/compressed/atags_to_fdt.c:84:13: error: assignment discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
   84 |         ptr = str - 1;
      |             ^
arch/arm/boot/compressed/atags_to_fdt.c: In function 'atags_to_fdt':
arch/arm/boot/compressed/atags_to_fdt.c:213:18: error: unused variable 'memsize' [-Werror=unused-variable]
  213 |         int ret, memsize;
      |                  ^~~~~~~
arch/arm/boot/compressed/atags_to_fdt.c:212:13: error: unused variable 'memcount' [-Werror=unused-variable]
  212 |         int memcount = 0;
      |             ^~~~~~~~
arch/arm/boot/compressed/atags_to_fdt.c:211:16: error: unused variable 'mem_reg_property' [-Werror=unused-variable]
  211 |         __be32 mem_reg_property[2 * 2 * NR_BANKS];
      |                ^~~~~~~~~~~~~~~~
arch/arm/boot/compressed/atags_to_fdt.c: At top level:
arch/arm/boot/compressed/atags_to_fdt.c:180:13: error: 'hex_str' defined but not used [-Werror=unused-function]
  180 | static void hex_str(char *out, uint32_t value)
      |             ^~~~~~~
arch/arm/boot/compressed/atags_to_fdt.c:63:17: error: 'get_cell_size' defined but not used [-Werror=unused-function]
   63 | static uint32_t get_cell_size(const void *fdt)
      |                 ^~~~~~~~~~~~~
arch/arm/boot/compressed/atags_to_fdt.c:43:12: error: 'setprop_cell' defined but not used [-Werror=unused-function]
   43 | static int setprop_cell(void *fdt, const char *node_path,
      |            ^~~~~~~~~~~~
arch/arm/boot/compressed/atags_to_fdt.c:25:12: error: 'setprop' defined but not used [-Werror=unused-function]
   25 | static int setprop(void *fdt, const char *node_path, const char *property,
      |            ^~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>