mdm2: Replace hsic platform_add/remove with connect/disconnect

mdm2 driver calls platform add/remove apis to notify hsic about
peripheral(9x15) connection/disconnection. Instead include hsic
connect/disconnect apis to keep mdm driver unaware of hsic
disconnection and discovery logic

Change-Id: I3bbb0d652d60de4bc5941e31eebbd75bb10b69b7
Signed-off-by: Vamsi Krishna <vskrishn@codeaurora.org>
Signed-off-by: Ramakrishna Prasad N <crpn@codeaurora.org>