spi_qsd: Config GPIOs by name rather then by 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 allows the driver to identify GPIOs by a
unique name, thus allows to specify any arbitrary
chipselect GPIO and reduces chances of simple
user configuration error.

Change-Id: I7cf1fb4329155bd5fc635257fc58d1dfa7312462
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>