riscv: replace macro DIM() with ARRAY_SIZE()

riscv: replace macro DIM() with ARRAY_SIZE()

OpenOCD already defines the macro ARRAY_SIZE, while riscv code
uses a local macro DIM.

Prefer using the macro ARRAY_SIZE() instead of DIM().
Not all the riscv code has been upstreamed, yes; this patch only
covers the code already upstreamed.

Change-Id: I89a58a6d91916d85c53ba5e4091b558271f8d618
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6258
Reviewed-by: Xiang W <wxjstz@126.com>
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>