pinctrl: sh-pfc: Make pinmux_cfg_reg.var_field_width[] variable-length

pinmux_cfg_reg.var_field_width[] is actually a variable-length array,
terminated by a zero, and counting at most r_width entries.
Usually the number of entries is much smaller than r_width, so the
ability to catch bugs at compile time through an "excess elements in
array initializer" warning is fairly limited.

Hence make the array variable-length, decreasing kernel size slightly.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>