mailbox, remoteproc: omap2+: fix compile testing

Selecting CONFIG_OMAP2PLUS_MBOX while compile testing
causes a build failure:

WARNING: unmet direct dependencies detected for OMAP2PLUS_MBOX
  Depends on [n]: MAILBOX [=y] && (ARCH_OMAP2PLUS || ARCH_K3)
  Selected by [m]:
  - TI_K3_M4_REMOTEPROC [=m] && REMOTEPROC [=y] && (ARCH_K3 || COMPILE_TEST [=y])

Using 'select' to force-enable another subsystem is generally
a mistake and causes problems such as this one, so change the
three drivers that link against this driver to use 'depends on'
instead, and ensure the driver itself can be compile tested
regardless of the platform.

When compile-testing without CONFIG_TI_SCI_PROTOCOL=m, there
is a chance for a link failure, so add a careful dependency
on that.

arm-linux-gnueabi-ld: drivers/remoteproc/ti_k3_m4_remoteproc.o: in function `k3_m4_rproc_probe':
ti_k3_m4_remoteproc.c:(.text.k3_m4_rproc_probe+0x76): undefined reference to `devm_ti_sci_get_by_phandle'

Fixes: ebcf9008 ("remoteproc: k3-m4: Add a remoteproc driver for M4F subsystem")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Andrew Davis <afd@ti.com>
Reviewed-by: Martyn Welch <martyn.welch@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>