arm/dt: msm8974: spi_qsd: Config GPIOs by name instead of index

Currently GPIOs are identified (MISO/MOSI/CLK or CS0..3)
by their index in the device-tree entry's array.
Further, chipselect GPIOs must be declared in order.
This allows to set a value for GPIO number three only
if the values for GPIOs one and two are also set.
This patch identifies GPIOs by a unique name, thus
allows to specify any arbitrary chipselect GPIO and
reduces chances of simple user configuration error.

Change-Id: Idd06607df2992d17a0ab9ad1e77b2db35ccd1d29
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>