pmdomain: rockchip: Fix build error

To resolve the build error with undefined reference to
`arm_smccc_1_1_get_conduit', let's add a build dependency to
HAVE_ARM_SMCCC_DISCOVERY.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 61eeb967 ("pmdomain: rockchip: Check if SMC could be handled by TA")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>