target/riscv: use coherent syntax in struct initialization

While initializing struct command_registration, the field's name "name"
is not specified, thus relying on the fact that it is the first field
declared in the struct and it's initialization value can be listed as
the first one.

Be coherent in the struct initialization and always use the field's
name.

Change-Id: Iefaeb15cc051db9f1e0f0140fe2f231b45f5bb12
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5013


Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>