mfd: rsmu_i2c: Convert to i2c's .probe_new() again

This commit reapplies commit 601e6d48 ("mfd: rsmu_i2c: Convert to
i2c's .probe_new()") which was accidently reverted by commit
1b3b1d6c27cc ("mfd: rsmu: Support 32-bit address space").

Without this change the driver fails to build in combination with commit
03c835f4 ("i2c: Switch .probe() to not take an id parameter") which
is contained in v6.3-rc2.

Fixes: 1b3b1d6c27cc ("mfd: rsmu: Support 32-bit address space")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230331070344.czphnnmvu2ojzo5l@pengutronix.de