spi: rzv2h-rspi: Fix max_speed_hz and clock configuration issues

spi: rzv2h-rspi: Fix max_speed_hz and clock configuration issues

Prabhakar <prabhakar.csengg@gmail.com> says:

This patch series addresses three issues in the RZV2H RSPI driver:
1. The max_speed_hz field was advertising a prohibited bit rate, which
   could lead to incorrect behavior when userspace applications attempt
   to set the SPI clock speed.
2. The clock configuration logic allowed for an invalid combination of
   SPR=0 and BRDV=0, which is not supported by the hardware.
3. Simplified  the clock rate search function as min/max speed parameters
   are not needed.

Note, patches apply on top of next-20260409.