target/cortex_a: fix compile error for uninitialized variable

target/cortex_a: fix compile error for uninitialized variable

Commit ad6c71e151590f9d07eb07eda978a8d2a845259c introduced the
variable "mmu_enabled" whose pointer is passed to cortex_a_mmu()
that initialises it.
This initialization is not visible to the compiler that issue
a compile error.
The same situation is common across the same file and the usual
workaround is to initialize it to zero; thus the same fix i
applied here.

Ticket: https://sourceforge.net/p/openocd/tickets/197/
Fixes: commit ad6c71e15159 ("cortex_a: fix virt2phys when mmu is disabled")
Change-Id: I77dec41acdf4c715b45ae37b72e36719d96d9283
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4619
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>