powerpc/fadump: define MIN_RMA in bytes rather than MB

The MIN_RMA size checks in fadump_setup_param_area() use
(MIN_RMA * 1024 * 1024), which is evaluated in int and can
overflow when MIN_RMA is increased to values such as SZ_2G,
triggering compiler warnings such as:

warning: integer overflow in expression of type 'int'
results in '0' [-Woverflow]

Define MIN_RMA directly in bytes using SZ_1M and update the
callers accordingly. This avoids repeated unit conversions and
prevents integer overflow.

Also convert MIN_RMA back to MB when populating the firmware
architecture vector, since firmware expects the value in MB.

Suggested-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/310b040acef712fdc79e3e37d0f4c2213938b556.1781589284.git.sayalip@linux.ibm.com