pinctrl: mvebu: count unnamed controls and allocate name buffer

pinctrl-mvebu allows SoCs to pass unnamed controls that will get an
auto-generated name of "mpp<PIN#>". Currently, we are allocating name
buffers on a per-control basis while looping over passed controls.
This counts the total number of unnamed controls and allocates a
global name buffer instead. The new buffer is then used while assigning
controls to pinctrl groups later.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>