MIPS: new Kconfig option ZBOOT_LOAD_ADDRESS

If this option is not 0x0, it will be used for zboot load address.
Otherwise, the result of calc_vmlinuz_load_addr will be used.

The zload-y value for generic are also removed then, as the current
value breaks booting on qemu -M boston.
The result of calc_vmlinuz_load_addr works well for most of cases.

The default value of bcm47xx keeps as it currently.

Signed-off-by: YunQiang Su <yunqiang.su@cipunited.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>