correct some missing-prototype warnings

This avoids the following warnings:
no previous prototype for 'bl2_arch_setup' [-Wmissing-prototypes]
no previous prototype for 'plat_log_get_prefix' [-Wmissing-prototypes]

Also correct a compilation issue if BL2_IN_XIP_MEM is enabled:
uintptr_t is not defined.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>