net: lpc-enet: factor out iram access

The lpc_eth driver uses a platform specific method to find
the internal sram. This prevents building it on other machines.

Rework to only use one function call and keep the other platform
internals where they belong. Ideally this would look up the
sram location from DT, but as this is a rarely used driver,
I want to keep the modifications to a minimum.

Link: https://lore.kernel.org/r/20190809144043.476786-7-arnd@arndb.de


Signed-off-by: Arnd Bergmann <arnd@arndb.de>