drivers: clk: agilex5: Configure intosc as boot_clk source

drivers: clk: agilex5: Configure intosc as boot_clk source

Some customers prefer to minimize the use of external oscillators,
especially when using the FPGA first configuration mode.

By enabling the configuration of the HPS internal oscillator as
the boot_clk source instead of the default external oscillator,
(HPS_OSC_CLK) in non-secure boot scenarios, this allows them
to eliminate the need for an additional oscillator device and
a dedicated HPS pin, simplifying board layout and routing.

Signed-off-by: Tingting Meng <tingting.meng@altera.com>
Signed-off-by: Alif Zakuan Yuslaimi <alif.zakuan.yuslaimi@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>