Fix an issue with multicast hash filters on Amlogic and Allwinner boards.

For DWC_GMAC_ALT_DESC implementations, the multicast hash table has only
64 entries. Instead of 8 registers starting at 0x500, a pair of registers
at 0x08 and 0x0c are used instead.

Approved by: re (hrs)
Submitted by: Guy Yur <guyyur@gmail.com>