stm32mp1: improve UART management in BL2

A reset_uart() function is created to improve readability in the function
bl2_el3_plat_arch_setup().
The secure/non-secure registering is removed as it is just stubbed in BL2.

Change-Id: Ib8d0319d3f4f54309848bc225b58608cea73bad9
Signed-off-by: Yann Gautier <yann.gautier@st.com>