serial: stm32: restrict _debug_uart_init() usage

serial: stm32: restrict _debug_uart_init() usage

Since commit 948da7773e34 ("arm: Add new config option ARCH_VERY_EARLY_INIT")
debug_uart_init() is called respectively in crt0.S and crt0_64.S.
That means that _debug_uart_init() is called for all STM32MP platforms
even for those which doesn't support SPL_BUILD.

So restrict _debug_uart_init() execution for platforms which can have
SPL_BUILD enabled (STM32MP1 platform only).

It's more needed to call debug_uart_init() in stm32mp1/cpu.c.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>