lib: sbi: Fix GET_F64_REG inline assembly

Current, GET_F64_REG() macro does not generate correct inline
assembly for the RV32 systems. This patch provides separate
definitions of GET_F64_REG() macro for RV32 and RV64 systems.

Signed-off-by: Charles Papon <charles.papon.90@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>