Three fixes to make LPUART1 work as NSH console on Nucleo-L552ZE-Q:
1. stm32l5_lowputc: add LPUART-specific BRR formula (256 * fCK / baud)
instead of reusing the standard USART divisor calculation.
2. stm32_boot: enable VDDIO2 and reconfigure LPUART1 GPIOs (PG7/PG8)
in board_initialize. The low-level setup runs before VDDIO2 is
enabled, so GPIOG writes silently fail.
3. stm32l5_serial: fix LPUART1 apbclock (PCLK1 not PCLK2), add
LPUART BRR branch in setformat, cast to uint64_t to prevent
32-bit overflow in (fCK << 8).
Also switch nsh defconfig from USART3 to LPUART1, add GPIO_LPUART1
pin definitions in board.h, and update board documentation.
Signed-off-by: anjiahao <anjiahao@xiaomi.com>