remoteproc: Use u64 type for boot_addr

elf64 entry is defined as a u64. Since boot_addr is used to store the
elf entry point, change boot_addr type to u64 to support both elf32
and elf64. In the same time, fix users that were using this variable.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Clement Leger <cleger@kalray.eu>
Link: https://lore.kernel.org/r/20200302093902.27849-4-cleger@kalray.eu


[bjorn: Fixes up return type of rproc_get_boot_addr()]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>