mfd: UCB1x00: Enable modular build

The UCB1200 / UCB1300 driver uses the MCP_SA11X0 driver, which
can be a loadable module, but this results in a link error
when UCB1200 itself is built-in:

drivers/built-in.o: In function `ucb1x00_io_set_dir':
:(.text+0x4a364): undefined reference to `mcp_reg_write'
drivers/built-in.o: In function `ucb1x00_io_write':
:(.text+0x4a3dc): undefined reference to `mcp_reg_write'
drivers/built-in.o: In function `ucb1x00_io_read':
:(.text+0x4a400): undefined reference to `mcp_reg_read'
drivers/built-in.o: In function `ucb1x00_adc_enable':
:(.text+0x4a460): undefined reference to `mcp_enable'
...

This can easily be resolved by making CONFIG_MCP_UCB1200 itself
a tristate option, since that causes Kconfig to track the
dependency correctly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>