board: sifive: unmatched: move kernel load address to 0x80200000

U-boot initially loads the kernel image to the kernel_addr_r, and
subsequently relocates it to memory address 0x80200000. Setting
kernel_addr_r to 0x80200000 can eliminate one copy operation.

Signed-off-by: Yong-Xuan Wang <yongxuan.wang@sifive.com>
Reviewed-by: Tom Rini <trini@konsulko.com>