pinctrl: renesas: Fix save/restore on SoCs with pull-down only pins

If some bits in a pin Pull-Up control register (PUPR) control pin
pull-down instead of pin pull-up, there are two pinmux_bias_reg entries:
a first one with the puen field filled in, listing pins with pull-up
functionality, and a second one with the pud field filled in, listing
pins with pull-down functionality.  On encountering the second entry,
where puen is NULL, the for-loop terminates early, causing the remaining
bias registers not to be saved/restored during PSCI system suspend.
Fortunately this does not trigger on any supported system yet, as PSCI
is only used on R-Car Gen3 and RZ/G2 systems, which all have separate
pin Pull-Enable (PUEN) and pin Pull-Up/Down control (PUD) registers.

Avoid this ever becoming a problem by treating pinmux_bias_reg.puen and
pinmux_bias_reg.pud the same.  Note that a register controlling both
pull-up and pull-down pins would be saved and restored twice, which is
harmless.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/59d2fbddff685b6a7a82ff17d2b37633e30e8860.1633615652.git.geert+renesas@glider.be

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