drivers: clock_control: stm32u5: Update condition on PLL1R values

Unlike stated in 57df225b396e43358aac4cc998ed2e99fdb57780, RM0456.pdf
reference manual mentions about PLL1R that "Only division by 1 and even
division factors are allowed."
Though, in reference manual, there is one issue on PLL1R values
description, which should actually be:
0000000: pll1_r_ck = vco1_ck
0000001: pll1_r_ck = vco1_ck / 2 (default after reset)
0000010: Not allowed
0000011: pll1_r_ck = vco1_ck / 4
...
This description will be fixed.

Reflect this in binding and driver.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>