media: aspeed: Rework memory mapping in probe

Use the recently introduced function devm_platform_ioremap_resource to
save a few lines of code. This makes the driver match common platform
device probe patterns.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Jae Hyun Yoo <jae.hyun.yoo@linux.intel.com>
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>