platform/chrome: fix wilco-ec dependencies

platform/chrome: fix wilco-ec dependencies

When CROS_EC_LPC is set to =m, we get a link failure for a
builtin wilco-ec module:

drivers/platform/chrome/wilco_ec/core.o: In function `wilco_ec_remove':
core.c:(.text+0x26): undefined reference to `cros_ec_lpc_mec_destroy'
drivers/platform/chrome/wilco_ec/core.o: In function `wilco_ec_probe':
core.c:(.text+0x18c): undefined reference to `cros_ec_lpc_mec_init'
core.c:(.text+0x224): undefined reference to `cros_ec_lpc_mec_destroy'
drivers/platform/chrome/wilco_ec/mailbox.o: In function `wilco_ec_mailbox':
mailbox.c:(.text+0x104): undefined reference to `cros_ec_lpc_io_bytes_mec'

The problem with the existing CROS_EC_LPC_MEC dependency is that this
is only for a 'bool' symbol, so the information about the exported
functions being in a module is lost on the way, and we actually have
to depend on both CROS_EC_LPC and CROS_EC_LPC_MEC.

Fixes: 7b3d4f44abf0 ("platform/chrome: Add new driver for Wilco EC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>