riscv: Fix efi header size for RV32

This patch depends on Atish's patch.
(https://patchwork.ozlabs.org/project/uboot/patch/20201013192331.3236458-1-atish.patra@wdc.com/)

Modify the size of the Optional Header "Windows-Specific Fields" to fit with the specification.
(https://docs.microsoft.com/en-us/windows/win32/debug/pe-format

)

Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Cc: rick@andestech.com
Cc: alankao@andestech.com
Cc: atish.patra@wdc.com
Cc: xypron.glpk@gmx.de
Cc: bmeng.cn@gmail.com