net: stmmac: dwmac-anarion: Use annotation __iomem for register base

Use __iomem annotation the register base: the ctl_block field of struct
anarion_gmac. I believe this is the normal practice for such variables.

By doing so some casting is avoided.
And sparse no longer reports:

 .../dwmac-anarion.c:29:23: warning: incorrect type in argument 1 (different address spaces)
 .../dwmac-anarion.c:29:23:    expected void const volatile [noderef] __iomem *addr
 .../dwmac-anarion.c:29:23:    got void *
 .../dwmac-anarion.c:34:22: warning: incorrect type in argument 2 (different address spaces)
 .../dwmac-anarion.c:34:22:    expected void volatile [noderef] __iomem *addr
 .../dwmac-anarion.c:34:22:    got void *

No functional change intended.
Compile tested only.

Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>