MFC r351184, r351189, r351217

r351184:
Add method for getting of syscon handle from parent device.

If simple multifuction device also provides syscon interface, its
childern should be able to consume it. Due to this:

• declare coresponding method in syscon interface
• implement it in simple multifunction device driver

r351189:
Fix bug introduced by r351184.
We should check the returned handle, not the pointer to it.

Noticed by: ian
X-MFC with: r351184

r351217:
arm64: a37x0_gpio: Use syscon instead of MMIO region

The fdt node for this driver is a simple-mfd and syscon compatible one
meaning that simplemfd will be the driver attached for it. The gpio driver
is attached to the 'gpio' subnode so use syscon_get_handle_default to
obtain the handle of the syscon from the parent device and use this
to read/write to the memory region.