spmi: pmic-arb: Use asynchronous probe

A spmi controller may have many child devices, which together may take
a signifigant amount of time to register and probe. Reduce this effect
by probing asynchronously.

After:
[    0.277217] calling__software_resume+0x0/0x24c_@_1
Before:
[    0.319295] calling__software_resume+0x0/0x24c_@_1

Change-Id: I95704a2ae7aae98e23dc9f29c7d023867d60388d
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>