mfd: rk808: Make MFD_RK8XX tristate

There is no reason for MFD_RK8XX to be bool, all drivers that depend on
it, or that select it, are tristate.

Fixes: c20e8c5b ("mfd: rk808: Split into core and i2c")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/d132363fc9228473e9e652b70a3761b94de32d70.1688475844.git.geert+renesas@glider.be


Signed-off-by: Lee Jones <lee@kernel.org>