mips32, dsp563xx: fix segfault on Gdb attach

mips32, dsp563xx: fix segfault on Gdb attach

Since c6216201b249e6a97fcc085e413e3d34e0de6fb7 gdb target description
generation support is enabled by default and it counts on checking
"feature" pointer in reg_list. Both mips32 and dsp563xx neither used
calloc nor explicitly set feature (as it was a newly introduced struct
field).

This patch changes all targets to use calloc for consistency.

Change-Id: I9eceadef8b04aacb108e24ae23cb51ca3009586f
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2102
Tested-by: jenkins
Reviewed-by: Salvador Arroyo <sarroyofdez@yahoo.es>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>