Revert "amba_pl011: Round input clock up"

This reverts commit 947ba6e85ea973b53e2f8d6860b191c0257e04fb.

Since [1], the clock driver has automatically rounded up the
UART clock frequency, so this patch is no longer needed.

[1] f690a4d7

 ("clk: bcm2835: Round UART input clock up")

Signed-off-by: Phil Elwell <phil@raspberrypi.com>