RISC-V: Separate elf_riscv_find_pbase out

The is the preparative patch for RISC-V kexec Image file support.

Separate the elf_riscv_find_pbase() function out to
allow kernel_load syscall load Image binary file.

Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Signed-off-by: Simon Horman <horms@kernel.org>