pinctrl: renesas: checker: Rework drive and bias pin iteration

The checker code to iterate over all drive strength and bias register
description items is cumbersome, due to the repeated calculation of
indices, and the use of hardcoded array sizes.  The latter was done
under the assumption they would never need to be changed, which turned
out to be false.

Increase readability by introducing helper macros to access drive
strength and bias register description items.
Increase maintainability by replacing hardcoded numbers by array sizes
calculated at compile-time.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/5051ae56a1388ccf2d283dfc9624de2991cce914.1650355619.git.geert+renesas@glider.be