mfd: simple-mfd-i2c: Add SpacemiT P1 support

mfd: simple-mfd-i2c: Add SpacemiT P1 support

Enable support for the RTC and regulators found in the SpacemiT P1
PMIC.  Support is implemented by the simple I2C MFD driver.

The P1 PMIC is normally implemented with the SpacemiT K1 SoC.  This
PMIC provides 6 buck converters and 12 LDO regulators.  It also
implements a switch, watchdog timer, real-time clock, and more.
Initially its RTC and regulators are supported.

Signed-off-by: Alex Elder <elder@riscstar.com>
Link: https://lore.kernel.org/r/20250825172057.163883-3-elder@riscstar.com
Signed-off-by: Lee Jones <lee@kernel.org>