include: sbi: Introduce common endianess conversion macro

Introduce cpu_to_lle and lle_to_cpu macros which invoke
correct word length cpu_to_le<64/32> conversion based on
__riscv_xlen.

Signed-off-by: Himanshu Chauhan <hchauhan@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>