ARC: kgdb: add 'fallthrough' to prevent a warning

Use the 'fallthrough' macro to document that this switch case
does indeed fall through to the next case.

../arch/arc/kernel/kgdb.c: In function 'kgdb_arch_handle_exception':
../arch/arc/kernel/kgdb.c:141:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
  141 |   if (kgdb_hex2long(&ptr, &addr))
      |      ^
../arch/arc/kernel/kgdb.c:144:2: note: here
  144 |  case 'D':
      |  ^~~~

Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>