emul: Make ESPI/I2C_EMUL depend on EMUL

The emulated espi and i2c bus controllers do not depend on CONFIG_EMUL
being enabled. When these fake bus controllers are enabled by their
assoicated compatible in the devicetree without CONFIG_EMUL it results in
linker failures for underlying peripheral emulators which may be difficult
to understand. The SPI config already depends on EMUL.

Make the ESPI and I2C emulated controllers depend on CONFIG_EMUL. Note:
This still allows linker failures if CONFIG_EMUL=n, but the linker failures
will be consistently at device driver instantiation due to a missing bus
controller. Regardless, this will be easier to debug when someone forgets
to enable CONFIG_EMUL.

Signed-off-by: Aaron Massey <aaronmassey@google.com>