mmc: sdhci: fix abort due to missing runtime PM

mmc: sdhci: fix abort due to missing runtime PM

When using i.MX ESDHC driver, while entering suspend while the device
is in runtime PM, the sdhci_(suspend|resume)_host function are called
with disabled clocks. Since this functions access the SDHC host
registers, this leads to an external abort on Vybrid SoC:

[   37.772967] Unhandled fault: imprecise external abort (0x1c06) at 0x76f5f000
[   37.780304] Internal error: : 1c06 [#1] ARM
[   37.784670] Modules linked in:
[   37.787908] CPU: 0 PID: 428 Comm: sh Not tainted 3.18.0-rc5-00119-geefd097-dirty #1540
[   37.796142] task: 8e246c00 ti: 8ca6c000 task.ti: 8ca6c000
[   37.801785] PC is at esdhc_writel_le+0x40/0xec
[   37.806431] LR is at sdhci_set_card_detection+0xe0/0xe4
[   37.811877] pc : [<803f0584>]    lr : [<803eaaa0>]    psr: 400f0013
[   37.811877] sp : 8ca6dd28  ip : 00000001  fp : 8ca6dd3c
[   37.823766] r10: 807a233c  r9 : 00000000  r8 : 8e8b7210
[   37.829194] r7 : 802d8a08  r6 : 8082e928  r5 : 00000000  r4 : 00000002
[   37.835974] r3 : 8ea34e90  r2 : 00000038  r1 : 00000000  r0 : 8ea32ac0
...

Clocks need to be enabled to access the registers. Fix the issue by
add runtime PM enabled pltfm implementation of suspend/resume which
take care of clocks by using the runtime PM API properly.