mailbox: ti-msgmgr: Use devm_platform_ioremap_resource_byname()

Convert platform_get_resource_byname() + devm_ioremap_resource() to a
single call to devm_platform_ioremap_resource_byname(), as this is
exactly what this function does.

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>