spi: rzv2h-rspi: Fix invalid SPR=0/BRDV=0 clock configuration

spi: rzv2h-rspi: Fix invalid SPR=0/BRDV=0 clock configuration

The combination of SPR=0 and BRDV=0 results in the minimum division
ratio of 2, producing the maximum possible bit rate for a given clock
source. This combination is not supported in two cases:

- On RZ/G3E, RZ/G3L, RZ/V2H(P) and RZ/V2N, RSPI_n_TCLK is fixed at
  200MHz, which would yield 100Mbps. The next hardware manual update
  will explicitly state that since the maximum frequency of the
  RSPICKn clock signal is 50MHz, settings with N=0 and n=0 resulting
  in 100Mbps are prohibited.

- On RZ/T2H and RZ/N2H, when PCLK (125MHz) is used as the clock
  source, SPR=0 and BRDV=0 is explicitly listed as unsupported in
  the hardware manual (Table 36.7).

Skip the SPR=0/BRDV=0 combination in rzv2h_rspi_find_rate_fixed() to
prevent the driver from selecting an invalid clock configuration on the
affected SoCs.

Additionally, remove the now redundant RSPI_SPBR_SPR_PCLK_MIN define
which was previously set to 1 to work around the PCLK restriction, but
was overly broad as it incorrectly blocked valid combinations such as
SPR=0/BRDV=1 (31.25Mbps on PCLK=125MHz).

Fixes: 8b61c8919dff ("spi: Add driver for the RZ/V2H(P) RSPI IP")
Fixes: 1ce3e8adc7d0 ("spi: rzv2h-rspi: add support for using PCLK for transfer clock")
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/20260410080517.2405700-3-prabhakar.mahadev-lad.rj@bp.renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>