remoteproc: Fix potential build issues with SPL remoteproc

The rproc uclass driver can either be built with SPL_REMOTEPROC
or REMOTEPROC, but the function prototypes in remoteproc.h are
defined only when CONFIG_REMOTEPROC is defined. This can cause
build issues in SPL if CONFIG_REMOTEPROC is not selected.

Fix this by replacing the existing precompiler macro usage with
CONFIG_IS_ENABLED.

Fixes: ddf56bc7 ("drivers: Introduce a simplified remoteproc framework")
Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>