clk: renesas: rzv2h: Use GENMASK for PLL fields

clk: renesas: rzv2h: Use GENMASK for PLL fields

Replace the older FIELD_GET-wrapping helper macros with plain GENMASK
definitions for the PLL CLK1/CLK2 field masks (CPG_PLL_CLK1_KDIV,
CPG_PLL_CLK1_MDIV, CPG_PLL_CLK1_PDIV and CPG_PLL_CLK2_SDIV). Update
rzv2h_cpg_pll_clk_recalc_rate() to explicitly extract those fields with
FIELD_GET and cast the KDIV extraction to s16 to ensure proper sign
extension when computing the PLL output rate.

Co-developed-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Fabrizio Castro <fabrizio.castro.jz@renesas.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/20251015192611.241920-3-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>