stmmac: platform: fix sparse warnings

This patch fixes the following sparse warnings. One is fixed by casting return
value to a return type of the function. The others by creating a specific
stmmac_platform.h which provides the bits related to the platform driver.

drivers/net/ethernet/stmicro/stmmac/dwmac-meson.c:59:29: warning: incorrect type in return expression (different address spaces)
drivers/net/ethernet/stmicro/stmmac/dwmac-meson.c:59:29:    expected void *
drivers/net/ethernet/stmicro/stmmac/dwmac-meson.c:59:29:    got void [noderef] <asn:2>*reg

drivers/net/ethernet/stmicro/stmmac/dwmac-meson.c:64:29: warning: symbol 'meson6_dwmac_data' was not declared. Should it be static?
drivers/net/ethernet/stmicro/stmmac/dwmac-sti.c:354:29: warning: symbol 'stih4xx_dwmac_data' was not declared. Should it be static?
drivers/net/ethernet/stmicro/stmmac/dwmac-sti.c:361:29: warning: symbol 'stid127_dwmac_data' was not declared. Should it be static?
drivers/net/ethernet/stmicro/stmmac/dwmac-sunxi.c:133:29: warning: symbol 'sun7i_gmac_data' was not declared. Should it be static?

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>