samples: ipc: openamp_rsc_table: Lower logging level

Having the log level set at debug causes a message to be printed
every time a mailbox message is sent or received. If we log this
using the RPMSG shell, then each message sent generates another
message, causing an an infinite message loop.

Do not set the level to debug by default, allow the level to
be set by the config LOG_DEFAULT_LEVEL.

Signed-off-by: Andrew Davis <afd@ti.com>