drivers: serial: pl011: Remove busy wait in Ambiq UART initiate

Ambiq UART requires specific busy wait during initialization for
propagating powering control registers, original k_busy_wait()
used here generated a dead loop because k_busy_wait() relays on
timer, who's driver is initialized after UART(UART init in
PRE_KERNEL_1, timer init in PRE_KERNEL_2), replace k_busy_wait()
with checking power status register is more suitable here.

Signed-off-by: Bryan Zhu <bzhu@ambiq.com>