make i2c-acorn tristate

It depends on tristate I2C and it's trivial to make modular.  The
current Kconfig allows I2C=m, I2C_ACORN=y, which doesn't work at
all; alternatives are dependency on I2C=y and making I2C_ACORN
itself a tristate.  The latter is the right thing to do...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>