remoteproc: Introduce sysfs_read_only flag

The remoteproc framework provides sysfs interfaces for changing
the firmware name and for starting/stopping a remote processor
through the sysfs files 'state' and 'firmware'. The 'coredump'
file is used to set the coredump configuration. The 'recovery'
sysfs file can also be used similarly to control the error recovery
state machine of a remoteproc. These interfaces are currently
allowed irrespective of how the remoteprocs were booted (like
remoteproc self auto-boot, remoteproc client-driven boot etc).
These interfaces can adversely affect a remoteproc and its clients
especially when a remoteproc is being controlled by a remoteproc
client driver(s). Also, not all remoteproc drivers may want to
support the sysfs interfaces by default.

Add support to make the remoteproc sysfs files read only by
introducing a state flag 'sysfs_read_only' that the individual
remoteproc drivers can set based on their usage needs. The default
behavior is to allow the sysfs operations as before.

Implement attribute_group->is_visible() to make the sysfs
entries read only when 'sysfs_read_only' flag is set.

Signed-off-by: Puranjay Mohan <p-mohan@ti.com>
Link: https://lore.kernel.org/r/20220216081224.9956-2-p-mohan@ti.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

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