mfd: abx500-core: Fix compiler warning larger stack frame

On systems with CONFIG_FRAME_WARN=1024, compiler warns the allocation of
an object of struct device on stack. Make the allocation dynamically to
fix the warning. Also change the caller's return type to int so as to
account for error handling.

drivers/mfd/abx500-core.c: In function ‘abx500_dump_all_banks’:
drivers/mfd/abx500-core.c:167:1: warning: the frame size of 1032 bytes
is larger than 1024 bytes [-Wframe-larger-than=]

Signed-off-by: Aurabindo J <mail@aurabindo.in>
Signed-off-by: Lee Jones <lee.jones@linaro.org>