net: broadcom: fix a mistake about ioremap resource

Commit d7a5502b ("net: broadcom: convert to
devm_platform_ioremap_resource_byname()") will broke this driver.
idm_base and nicpm_base were optional, after this change, they are
mandatory. it will probe fails with -22 when the dtb doesn't have them
defined. so revert part of this commit and make idm_base and nicpm_base
as optional.

Fixes: d7a5502b

 ("net: broadcom: convert to devm_platform_ioremap_resource_byname()")
Reported-by: Jonathan Richardson <jonathan.richardson@broadcom.com>
Cc: Scott Branden <scott.branden@broadcom.com>
Cc: Ray Jui <ray.jui@broadcom.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Dejin Zheng <zhengdejin5@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>