Change the default partition mode to A_Mode

Traditional IBM systems since 1988 used to ship with A_Mode
as the default partition mode for iBMi. Changing the default
value from D_Mode to A_Mode to match the behaviour of our systems
and align them with the earlier behaviour.

Change-Id: I4fed505de3cf5628e5ad668870b6fd1ac00dc588
Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>