pinctrl: renesas: rzg2l: Fix SMT register cache handling

Store SMT register cache per bank instead of using a single array.

On RZ/V2H(P), RZ/V2N, and RZ/G3E, the SMT register is split across two
32-bit registers: bits 0/8/16/24 control pins 0-3, while pins 4-7 are
controlled by the corresponding bits in the next register.  The previous
implementation cached only a single SMT register, leading to incomplete
save/restore of SMT state.

Convert cache->smt to a per-bank array and allocate storage for both
halves.  Update suspend/resume handling to save and restore both SMT
registers when present.

Fixes: 837afa592c623 ("pinctrl: renesas: rzg2l: Add suspend/resume support for Schmitt control registers")
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20260413182456.811543-2-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>