microblaze: Disable all cpu features before reset

Fix microblaze soft reset function and disable
all cpu features. Especially disable caches because
IRQs were off by disable_interrupts().

Reported-by: John Williams <john.williams@xilinx.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>