stm32mp1: move stm32_get_gpio_bank_clock() to private file

GPIOx clocks are specific to each STM32MP platforms.
This change moves function stm32_get_gpio_bank_clock() from stm32mp
common source files to platform private stm32mp1_private.c source file.

Change-Id: I9616c0d3fe4d10af715d6f2d1550c13ab62c829a
Signed-off-by: Yann Gautier <yann.gautier@st.com>