Add custom I2C accessors to this driver, since the regular I2C regmap ones
do not generate the exact I2C transfers required by the chip. On I2C write,
it is mandatory to send transfer length first, on read the chip returns the
transfer length in first byte. Instead of always reading back 8 bytes, which
is the default and also the size of the entire register file, set BCP register
to 1 to read out 1 byte which is less wasteful.

Fixes: 892e0dde ("clk: rs9: Add Renesas 9-series PCIe clock generator driver")
Reported-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20220929195521.284497-1-marex@denx.deReviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>